MS Access DataBase Topics

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MS Access Data Base and Structured Query Language (SQL)

1. Motivation and Context

Though databases are a topic of general interest in computing and informationtechnology, our specific purpose in introducing them here is to obtain enough familiarity

with them in order to implement the pervasive Internet application architecture called the

three-tier architecture. The three-tier architecture is a software architecture model thatencompasses:

1. A clientweb browser interacting with

2. a webservercapable of executing programs like JSP where3. the server in turn mediates or interfaces with a background database such as

MS Access

Thus, the server acts as an intermediary between the client(s) and database(s). This is theessential paradigmatic software architecture for Internet ecommerce applications. We

have already seen how clients and servers behave. We shall now turn our attention todescribing the rudiments of databases using the MS Access Database for convenience.

MS Access provides a convenient graphical interface for invoking its capabilities or

functions, as well as allowing you to use the Structured Query Language (SQL) forquerying the data base. SQL is a standard syntax for formulating queries to a database

and is generally portable across different database platforms. Understanding SQL, in

addition to the manual GUI interface that Access provides, is important because of its

widespread use as well as because we will subsequently interface with the data base froma Java Server Page (JSP) program which precludes using the GUI interface and can only

be assessed using SQL commands. The JSPs that execute on the server can respond toHTML-Form-driven requests from the client, using the client's Form data to interact withthe database. This kind of program-driven interaction with the database is essential to

any large scale web application because in such cases the number of interactions

precludes their being handled manually.

The objectives of this section are to understand how to:

1. Create and populate a data base in Access.2. Define basic SQL syntax using Access and use built-in Arithmetic Functions.

3. Formulate basic SQL queries.

4. Formulate Queries involving multiple tables: table joins.5. Use the MS Access graphical tool for creating queries.

6. Use additional miscellaneous capabilities and SQL queries such as establishing

password security for a database, SQL data deletion and insertion, creating tableswith SQL, the IN operator for sets, and nested queries.


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2. Create and Populate a Database Table

Select MS Access from the start menu. Select the blank data base option in thedialog window and navigate to where you want to save the database. [This option is on

right-hand-side panel, not on the toolbar.] Save it as example00.mdb. Refer to web site

linkAccess DB examples > example00.mdb for a copy of completed example. At thispoint the screen looks something like:

Access is a table-structured data base. A table has an attribute or property in each

of its column. The rows of the table represent instances of entities with their attributes

given. This is best illustrated by an example of a populated table.

To create a Table in the data base, select the Tables option under the Objectsheading in the dialog window. Then select and double-click the 'Create table in Design

View' icon option. Define the attributes for the table in the grid-dialog window that

appears, as well as the data types of each of the attributes. See the snapshot below. Thedata type options are selected from theData Type column. In the example used, some of

the attributes are naturally TextorNumber, but the cost is naturally of type Currency.
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Observe that this is not where you populate the database with data. This is where you

define the attributes or properties of the database table and their data types. This is

definitional information about the table, not the contents of the table. It is meta-data asopposed to data.

If you go to close the Table [by clicking the X in upper right of window], you will

be given a choice of saving the Table. At this point, provide a table name ('Books') in the

dialog window that opens. After this, MS Access will also ask whether you want toidentify a search key. A search key is an attribute whose value is unique: it appears in

only one row of the table. The purpose of a search key is twofold: it uniquely identifies

records in a table; it also speeds up searching for records since when keys exist for atable, Access can internally organize the table data to expedite locating records/rows with

given key values. Familiar examples of search keys are student ids or a social securitynumber. If you sayNo to Access's request to define a search key, none will be associated

with the table. If you define one of the attributes of a table to be a key, Access willprevent you from saving rows (records) which duplicate the key value of any existing

record in the table. To declare an attribute a key, right-click the leftmost column just to

the left of the intended name of the attribute and select theprimary key option in the pop-down menu that appears. Alternatively, you can let Access automatically create an ID

key for a table. For example, if you say 'Yes' when you close the table property


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definition the first time, Access provides an automatically generated 'ID' with values

starting at 1 and automatically updated sequentially and so acts as a key. When the table

definition is closed, you can re-open it by clicking on the table icon and right-clickingthe title bar to select the design view. You can also switch back to the data by selecting

DataView the same way. If you selected the automatically generated ID alternative, then

the screen would look like the following:

To populate the table that is insert data in its rows/records - just enter data in

the attribute fields. The default ID values will be generated automatically: Access will

prevent you from entering data in the ID field since Access itself generates theseautomatic key values. To enter a new row of data, just enter data in the bottom, open or

empty row. For example:


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To review or revise the table attributes or properties, select Tables under the

Objects column, highlight the table (Books) icon of interest, then click theDesign icon onthe toolbar of the window. Alternatively, right-click the opened data table on its blue title

bar and select Design in the pop-down menu that appears. This re-opens the Properties

window which lets you change the table properties and data types.

3. Defining SQL Queries in Access and Built-in Arithmetic Functions

SQL is a standardized way to query or ask a question about what's in a data base.The same kind of syntax is used in many different database systems like Access, Oracle,

MySQL, etc. It is essential to understand SQL if you want to be able to embed queries in

a programming language. We will do this later when we retrieve data from a database byembedding SQL queries in a JSP program. The combination of SQL with a

programming language provides a powerful tool which allows one to flexibly process the

information in a database. The following examples are more 'hardwired' than queries in aJSP. Here, the attribute values are explicitly specified, while in a program they would

usually be variables.

The SQL queries utilize the following core syntax:

SELECT the data or attributes wanted for the answer

FROM database tables needed to handle the query [together with

temporary variable names for the tables]


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WHERE Boolean restrictions are specified which depend on the

question, as well as

interlinking-conditions - for the tables needed to answer the

query based on their shared attributes

To define an SQL query in Access, click Queries under Objects in the dialog

window, then click'Create query in Design view'. For simplicity, close the 'show table'dialog window' that opens, but leave the 'Select Query'window open. Click the SQL tab

on the uppermost, top left toolbar for Access (the outermost window you are working

with) and select SQL View on the pop-down menu, which will clear the 'Select Query'

window so it just contains 'Select;'. Then enter the desired SQL query in this text area.For example, you can begin with a query that retrieves all (the information on) the Books

where the author is 'McHugh' as follows:

Select *From Books as b

Where b.author = 'McHugh'

In this example, the notation '*' means that all the attribute values of the rows that satisfy

the Where condition are to be returned or displayed. In this example, the only tableinvolved is Books, which we have given the shorthand name b (just like an algebraic

variable.) In order to execute the query, click the red exclamation point ( ! ) that appears

after you have entered the query on the topmost Access toolbar, to execute query. (This

requires the Query dialog window to have had at least a entered in order for the !to be accessible.)


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The query returns the requested results in a table as shown below. To see the original

query again, right click the data table on the title bar and select SQL View. If you closethe Query-1 window, you will be asked at that point if you want to save it (for future

reference). It is important to verify the correctness of the query results certainly at least

when the query is first being tested. It is easy to incorrectly define a query that looks orappears to right because it gives the right answers. But the query may actually be

wrongly formulated. For example, it may also give extraneous answers in addition to the

correct ones, or it may omits some of the correct answers. Remember that the answer isonly correct if it supplies the whole truth and nothing but the truth. Supplying some of

the correct answers is not enough; adding in some incorrect answers as well won't do

either.


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Toselectively retrieve attributes, we list the desired attributes in the Select clause:

Select title, costFrom Books as b


In this case a two-column table with title & costattributes is returned, with rows that

satisfy the Where clause condition. Caveat: Because the Access data font is small, it's

easy to inadvertently introduce, for example, a leading blank before the data (such as intyping ' McHugh' by mistake). In such a case, the string ' McHugh' would not match the

string 'McHugh' (since the actual data presumably does not have the leading blank), so an

empty table would be returned. It is also easy to overlook having introduced a space

before the dotin expressions like b.author (such as b .author). This will trigger a popupprompt for a parameter value which is actually irrelevant. Misspelling attribute names

also triggers the same indirect manifestation of an error.

To retrieve all authors other than 'McHugh' use theNotoperator:

Select title, cost

From Books as bWhere Not(b.author = 'McHugh')

which means the same as: author is not 'McHugh'.

The Where clause can also be omitted as in the following query:

Select title, author


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From Books as b

in which case all the title/author entries in the table are retrieved without qualification.

The syntax for using mathematical formulas in is easy and natural. For example,

if you want to retrieve the dollar value of the inventoryper bookwhere 'McHugh' is theauthor, the costper book and the numberof books should be multiplied as in:

Select title, b.cost * b.stockFrom Books as b


To determine the total value of the inventory over all such books, we use the built-in

function Sum applied to the formula:

Select Sum ( b.cost * b.stock )

From Books as bWhere b.author = 'McHugh'

which returns the results in the snapshot shown below. Additional examples of

arithmetic calculations are given in the baseball database below.The MS Access arithmetic capability can be embedded in JSP programs to

facilitate accounting style calculations by relying on built-in services provided by Access.

This is quite important since it means that you can let SQL queries embedded in your JSPprogram perform potentially complex calculations without having to write the algorithms

yourself in Java. The SQL lets you obtain these effects using simple imperative

descriptions of what you want to calculate with almost no effort on your part except theformulation of the SQL statement.


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4. More SQL Queries

A Where conditions can be a combination ofBoolean expressions. For example,

the following query retrieves entries with author 'McHugh' or title 'Physics'.

Select *

From Books as b

Where b.author = 'McHugh' or b.title = 'Physics'

The Where clause can also specify a very useful approximate search. It uses a JavaScriptstyle of pattern matching syntax. As an example, the following query retrieves titles that

begin with the letter 'I' - followed by an arbitrary length sequence of other characters.

Select *

From Books as bWhere b.title LIKE 'I*'

The '*' in the pattern denotes zero or more wildcard characters. On th other hand, thenotation ? denotes any single character, that is, it matches any character.

Note: When using this SQL notation in Javause % instead of * and _ instead of ? .

This kind of approximate or pattern-based search capability is very important inmany applications including ecommerce. It permits a Java Server Program to incorporate

embedded SQL that can provide powerful search capabilities in Select queries. Consider,

for example, an application for a site that sells books. An approximate search makes ittrivial to retrieve books not just on the basis of their complete and exact titles, but also to

retrieve titles on the basis of user-supplied search fragments. Furthermore, this capability

is immediately available without any algorithmic development required on the part of the


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JSP programmer. All that is required is the formulation of an appropriate SQL query

which entails only a problem-solving-like formulation, not algorithmic programming. At

least as important is the fact that the algorithms which are built in to Access that providethis ability are already thoroughly tested and verified in detail as part of that package.

The algorithms do not have to be developed, tested, or verified at the JSP stage.

Particularly in the case where a single attribute is being retrieved, the Where

conditions in a query may select duplicate rows. For example, the query "Select author

From Books as b Where b.author = 'McHugh'" returns every book with 'McHugh' as anauthor. It is simple to remove duplicates by including the key wordDistinctin the select

clause as follows:

Select DISTINCT authorFrom Books as b


5. Queries Involving Multiple Tables - Table Joins

So far, the examples we have considered involve a single table. However,

generally a database contains multiple tables with the entries in one table related to

entries in the other tables. To illustrate this, we have added another table to the sampledatabase. We will denote the table asPublishers[name, address, phone, code] which is a

standard notation for referring to a table and its attributes. Let us consider a query whose

result depends on both tables. Suppose we want to retrieve the addresses of those

publishers who publish McHugh's books. The obvious query that springs to mind is:

Select p.address

From Books AS b, Publisher as pWhere b.author = 'McHugh'

However, the problem is that this query will retrieve the addresses of all three publisherslisted in the Publishers table, even though only two of the Publishers actually satisfy the

query. To understand what is happening, let us first modify the query to display all the

attribute values retrieved: "Select * From Books AS b, Publisher as p Where b.author =

'McHugh'". This allows us to better understand what we selected. The retrieved table isshown below.


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The retrieved table [Query1] shown here has been sorted on author. Notice that

each of the rows for 'McHugh' (from the Book table) has had pasted onto it a copy ofevery row from the Publishers table. One of these rows introduces an extraneous

publisher. Why is this occurring? The reason is that Access first automatically takes all

possible combinations of the rows of the two tables - and only then extracts the rows with'McHugh' as author. Viewed more abstractly, suppose we let A, B, C, D, and E denotethe five rows from the Books table and let 1, 2, and 3 denote the three rows in the

Publishers table. The so-called Cartesian productof the two tables is:

{ A1, A2, A3, B1, B2, B3, C1, C2, C3, D1, D2, D3, E1, E2, E3 }

whereXi represents the concatenation of row X from Books and row j from Publishers.This intermediate result is created by Access before the Boolean Where condition

(b.author = 'McHugh') is applied. Since 'McHugh' occurs in the Rows A and D (in the

Books table), then the selected rows after the condition (b.author = 'McHugh') is applied

will be:

{ A1, A2, A3, D1, D2, D3 } .

Two of these entries are completely extraneous to our intended query. They only arise

because every Publisher row has been pasted on every Book author row, regardless of

whether there was any relation between the rows. The following discussion shows howto avoid this byjoiningthe tables properly.


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The question is how to join tables so as to eliminate extraneous combinations. The

way to do this is to use additional conditions in the Where condition that preventextraneous rows from being combined. In the present example, the publisherCode

attribute in the Publishers table should have the same value as the code attribute in the

Books table whenever we combine the information in the tables; otherwise it's likecombining apples and oranges. We can force this by including linking conditions that let

Access combine table rows only when the codes in the combined rows match. The

correct query is:

Select p.address

From Books AS b, Publisher as p

Where b.author = 'McHugh' and b.publisherCode = p.Code

This returns the Query1 table shown below. In the combined rows that appear in that

table, the publishers for 'McHugh' have matching codes 11 and 444 (and addresses NY

and NJ) as expected.

Further Table Join Examples

The next few examples use a simple data base for baseball teams. The table

teamTable has attributes teamName [text] and teamId [text]. The table managerTablehas attributes manager [text], the town where the team lives, and his teamId. A third


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table playerTable table has attributes name, teamId, and home, corresponding to the

player's name, his teamIdattribute (which is common to the three tables), and the player's

hometown. The tables are populated as shown.

teamTable managerTable playerTable

When following this, use the same values as here so that yourqueries have the same results as those we describe. The data is extensive enough so we

can accurately test whether queries are really finding out what we think they are.

Remember you have to carefully verify the results are fully correct, not just partiallycorrect (lacking some results, or containing superfluous results). Most of the examples

are recorded in Access DB examples > exp04.mdb. The following problems all require

queries that use more than one table. We have expressed the questions as natural

language (English) statements, so it requires some thinking to see how to formulate themas SQL queries.

i. Find in what town the yankees play.

The teamTable does not directly name the team's town only the teamId.

However, the teamTable does associate a teamName with the teamId. Thus, the queryentails combining two tables using a linking condition based on teamId:

SELECT ... to be filled in ...

FROM managerTable AS m, teamTable AS t

WHERE t.teamId=m.teamId AND ... to be filled in ...

The rest of the Where condition has to identify that we are interested in the manager oftheyankees, and the Selectclause has to identify the expected results; namely, the

manager's town m.town. The completed query is:

SELECT m.town

FROM managerTable AS m, teamTable AS t

WHERE t.teamId=m.teamId AND m.teamName='yankees'

teamName teamId

yankees 1

dodgers 2

giants 3

managerteamI

dtown

n1 1 NY

n2 2 LA

n3 3 Pittsburgh

n4 4 SF

name teamId home

p11 1 NY

p12 1 NY

p13 1 LA

p21 2 NY

p22 2 LA

p23 3 LA

p31 3 LA

p32 3 LA

p41 4 SF

p42 4 SF
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Here the teamName is quoted (in single quotes) because teamName is of type text. Had it

been of type number, the quotes would have been omitted. Incidentally, if you enter an

incorrect attribute for a table (say, m.name instead of m.town), the system will respondwith a request for a parameter value.

ii. Find all the Yankee players who live in the same town as their manager.

The question does not have an immediately obvious SQL formulation.

Furthermore, to answer the question, you have to link together all the tables it requires:

the managerTable, teamTable, and playerTable. Omitting any of these leads toextraneous results. For example, if you formulate the query as:

SELECT p.name

FROM managerTable AS m, playerTable AS p, teamTable AS t

WHERE t.teamId=m.teamId

AND p.home=m.town AND teamName='yankees'you pick up an extraneous player (from the dodgers, team 2: p21) because the tables were

not properly joined. Since there are three tables with shared attribute teamId, all threehave to be linked, requiring two linking conditions as follows:

SELECT p.name

FROM managerTable AS m, playerTable AS p, teamTable AS t

WHERE t.teamId=m.teamId AND m.teamId=p.teamId

AND p.home=m.town AND teamName='yankees'

iii. Find the team [by name] which has a manager who lives in Pittsburgh.

Various attempted queries are illustrated in Query 10-11 in exp04.mdb; query 10

illustrates a misspelling.

iv. Find all the towns where a manager lives but some (nonzero number) of that

manager's players do not live in that same town.

6. Not Operator, avg, min, max Functions

It's helpful to first understand the query by examining the data provided bymerging the manager and player data:

SELECT *

FROM managerTable m, playerTable pWHERE m.teamId = p.teamId


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This correctly combines manager information with the data for each player on the

manager's team (as opposed to merely merging rows that should not be matched up - like

managers with players from other teams). You can use these results to identify whichplayers live in different towns from their managers. Given this, you can verify the

correctness of the following where we have added the additional constraint that the

manager and player live in different places using the BooleanNOToperator:

SELECT *

FROM managerTable m, playerTable pWHERE m.teamId = p.teamId

AND NOT( p.home = m.town )

The following example illustrates how built-in SQL mathematical functions can be

combined. Suppose we have added new attributes to teamTable for average player

salary (of type currency) and number of players per team (numPlayers, of type number).

Verify that the following query calculates the average salary per players over all teamstaken together:

SELECT sum ( numPlayers * salary ) / sum (numPlayers )

FROM teamTable

The formula calculates the total outlay for salaries, divided by the total number of players

on all the teams. You could also try to calculate the average salary using the built-in

function avg:

SELECT avg ( salary )

FROM teamTable

but this would not calculate the overall average correctly since the overall average is a

weighted average. Other functions include: max, min.

7. Access Graphical Tool for Defining Queries

MS Access also provides a graphical tool for creating SQL queries which doesnot require explicit awareness of SQL syntax. For example, suppose you want to

determine the addresses of all the publishers that author 'McHugh' publishes for and

which have the digits '33' somewhere in their phone number. The SQL query would looklike:

Select p.address , p.phone

From Books as b, Publishers as pWhere b.author = 'McHugh'

AND b.publisherCode = p.code

AND p.phone LIKE '*33'


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We could also define this query graphically as follows:

1. ClickCreate query in Design View.

2. Use the Show-Table window that appears and click theAddbutton to add the

(selected) tables you want toJoin together. The table-icons will appear in the upper

section of the dialog window along with a scrolling view of their attribute names.3. To link (Join) a pair of the icon-tables (like on the shared attributes

publisherCode and Code), highlight one of the attributes [say,publisherCode] in one of

the tables and dragit over the corresponding attribute [say, Code] in the other table,producing a two-way arrow between the linked attributes. Note:In this example, delete

the default arrow between the automatically generated ID attributes since they don't

make sense in this problem and impose irrelevant restrictions on the table entries. Thetables can then be merged using run (denoted by the red exclamation point ! ).

4. To graphically define Where criteria click in one of the fields at the bottom of

the dialog window and select a desired attribute from the resulting pull-down menu. To

define a Boolean condition, enter the right-hand-side of the intended condition for the

Where condition in the Criteria field as illustrated in the example, for example: Like '*33'

for the phone number condition, which is equivalent to the condition:

Where p.phone = '*33' .

This can be repeated for multiple attributes with the resulting conditions then And' ed.

If you enter the Criterion in the "or" field instead, it acts like an OR rather than an AND.

5. To make an attribute appear in the retrieved table, click the Show checkbox.

6. Finally, clickrun [ ! ] to see the resulting table of selected attributes.

For a related simple example, do a join on the two tables with the shared code as the

linked field. The resulting table contains only consistent data rather than incompatible

extraneous data items.

Another example of the Graphical query tool is for the following baseball

example. The query is:

Find all the Yankee players who live in the same town as their manager.

Using the Show table tool underCreate query in Design view, add the managerTable andtheplayerTable. Link them on the shared teamIdattribute as before. The same thing can

be done with other attributes as well that may not be shared but which you want to force

to be equal. For example, also link the two tables on the town and home attribute as well,since these are expected to be the same: this way we do not have to explicitly set up that

query criterion. Additionally, force the teamName to be the 'yankees' using a Field

criteria. Then, select what you want to have shown in the result table, let's say the

player's names. These are selected at the bottom of the window using Show checkbox.The snapshot shows the setup. If you want to require a negative condition, say that the

team is not the Yankees, you can use not('yankees) or 'Yankees' as the criterion.


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8. Additional MS Access Functionality

8.1 Database Password Protection

You can easily provide password protection for an Access database. First closeany open copy of the database. Then, re-open MS Access, but not the sample data base

itself. Then, select File > Open in the Access menu, navigate to the directory with the fileto be secured, and select or highlight (but don'tdouble-click or open) the desired database

file. The open-menu-button on the bottom right of the window (see the figure below):

will open a pull-down menu: select the 'open exclusive' option.


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Then, select the Tools > security> Set database password option in the database dialogwindow toolbar (below) and set the password in the resulting dialog window.

If you wish to remove password protection, again, first close Access. Then re-open it with the 'open-exclusive' option described previously. Then, select Tools >

security> unset database password to remove the password. You will asked for the

password again when you try to change the password, even though you already gave the

password when you opened the database in the first place. Arguably, this second requestis appropriate. For example, you may have walked away from your computer after

initially opening it. You would not want someone to be able to interlope and change the

password at that point without the screening test of the additional password request.


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8.2 Delete or Insert Rows in Tables and Create new Tables

The following commands are more likely to be used in the context of a JSP

program. For example, the Insert command would probably be used with variables

whose value[s] were obtained from an HTML Form. So far we have obtained the sameeffects using the Access interface for making tables and entering values directly into

tables, but in programming applications the GUI interface of Access is unavailable.

Furthermore the GUI interface primarily uses fixed rather than variable expressions.

a. Insert a row in a table:

Insert Into Publishers Values ( '6', 'Auerbach', 'NY' , '212-777-7777', '99')

This inserts a row of data into the Publishers table. If the SQL occurs in Java

statement the single quotes ( ' ) have to be included. The number of values provided in

the list has match the number of attributes in the table. In this example,Publishers alsohas an automatically generated key ID. The current maximum value ofID is 5, so the

next ID should be '6', though the keys do not have to be in sequential order, just non-

redundant. If a copy of the table is already open, the insert will not be immediatelyapparent. You have to close the table and reopen it to see the inserted row. If you try to

add a row where theID key duplicates an existing key value, the request will be blocked.

b. Deleting rows from a table:


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The syntax fordelete is as illustrated:

Delete From Publishers Where ID = 12 ;

This removes the row withID=12 from the indicated table. The attributeID is a numberso it is not enclosed in quotes. If a statement like:

Delete From Publishers Where name = 'Thomson'

were used, where the attribute is not a unique key, then allrows satisfying name =

'Thomson' are deleted.

c. Creating a new table:

Create table table1 ( ssn integer , name char(22) )

This creates a new table named table1 with the indicated attributesssn and name.

The attributes must be given data types. The type integerforssn makes ssn of typenumber. The type char(22) forname makes name of type text, with the field size for the

text attribute in this case being 22 characters. You can verify this under the Table Design

view oftable1's properties. Select the name attribute or the text Data Type for the nameattribute: theField Properties at the bottom of the window displays the name field

characteristics. The same applies to the numberproperty.

8.3 Set OperatorINand Nested Queries

The BooleanINoperator acts like a set membership test or operator. Thus,property IN ( 'x', 'y', 'z' ) is true if theproperty or attribute's value is 'x' or 'y' or 'z'. The

INcan also be thought of as replacing a number of OR conditions. The following query

illustrates the syntax. It returns all titles where the book author is 'McHugh' or'Archimedes'.

Select b.title , p.name, b.author

From Publishers as p, Books as b

Where b.authorIN( 'McHugh' , 'Archimedes' )

and

p.code = b.publisherCode

The second part of the Where condition links the two tables so the Join is done correctly.

TheINcondition is equivalent to the OR statement:

b.author = 'McHugh' OR b.author = 'Archimedes'


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TheINoperator works nicely with what are called nested queries. One merely

replaces the list of values inside theINoperator's parentheses with an SQL query which

returns a single-attribute-table of values. For example:

Select b.author, b.title

From Publishers as p, Books as b

Where p.code IN ( Select p.codeFrom Publishers p

Where p.address IN ( 'NY', 'NJ' )

)

ANDp.code = b.publisherCode

Notice that the structure of the IN clause is: property-x IN ( Select property-x From ...)

with the same property occurring in both places. Thus this Where condition is equivalentto restricting the codes to those of publishers located in NY or NJ. The outer Select then

uses these codes to identify authors who work with those publishers.

Construction Notes

1. find teams of players who live in ny but their managers don't - dodgers2. select distinct teamName

from teamTable as t , managerTable as m, playerTable as p

where not ( m.town = 'ny' ) and p.teamID=m.teamId and p.teamId=t.teamId

and p.name IN ( select name from playerTable as p where p.home = 'ny' )3.better nested example

4. prompts & parameter

Documents

MS Access DataBase Topics