Basics of Database Tuning - Computing · PDF fileCMPT 354: Database I -- Embedded SQL 2 SQL Is Not for All • SQL is capable in manipulating relational data • SQL is not good for

Embedded SQL

CMPT 354: Database I -- Embedded SQL 2

SQL Is Not for All

• SQL is capable in manipulating relational data• SQL is not good for many other tasks

– Control structures: loops, conditional branches, …– Advanced data structures: trees, arrays, …– Not good for general problem solving

• How to solve a general problem involving relational data?– Use a general programming language (e.g., C++/Java)

in general but use SQL for relational data access– How to embed SQL into a general programming

language?


Integrating SQL and Programming?

• Typical procedural programming languages– C/C++, Java– Programs = data structures + algorithms– Structured data variables: records, arrays– Procedural semantics: loop, control structures

• SQL– Sets: query results are sets, unordered in

general– No control structure in general– Hiding data structures and algorithms


Database management

system

Programming Environment

program

files

SQL

database

storage

?


Database management

system

SQL and Hosting Language

program

files

SQL

database

storage

embedded


Embedding SQL

Host language+

Embedded SQL

preprocessor

Host language+

Function calls

Host-language compiler

Object-code program

SQL library

Pre-compile


SQL/Host Language Statement?• How can a preprocessor tell whether a statement is for

SQL or for the host language?– Keywords EXEC SQL in front of the statement

• How can an SQL statement use a variable in the host language?– Insert a new customer whose information is input by a userchar customer_name[20], customer_street[30], customer_city[30],

SQLSTATE[6];… //host language stuffEXEC SQL BEGIN DECLARE SECTION;

char customer_name[20], customer_street[30], customer_city[30];char SQLSTATE[6];

EXEC SQL END DECLARE SECTION;– Declare a host variable in SQL before using it in SQL statements


Embedded SQL

• SQL can be embedded in many host languages such as C/C++, Java, and COBOL

• EXEC SQL statement is used to identify embedded SQL request to the preprocessor– The basic form follows that of the System R embedding

of SQL into PL/IEXEC SQL <embedded SQL statement>END_EXEC– May vary by language (for example, the Java

embedding uses # SQL { …. }; )


Using Shared VariablesEXEC SQL BEGIN DECLARE SECTION;

char customer_name[20], customer_street[30], customer_city[30];char SQLSTATE[6];

EXEC SQL END DECLARE SECTION;customer_name=get_customer_name();customer_street=get_customer_street();customer_city=get_customer_city();EXEC SQL INSERT INTO customer(customer_name,

customer_street, customer_city)VALUES (:customer_name, :customer_street, :customer_city);

• The shared variables are prefixed by a colon in SQL statements– They appear without the colon in host language statements


Status of SQL Statement Execution

• How can we know whether an update succeeds or not?– SQLSTATE connects the host language

program with the SQL execution system• SQLSTATE: an array of five characters

– Each time a function of the SQL library is called, a code is put in SQLSTATE

– ‘00000’: no error condition occurred– ‘02000’: a tuple requested is not found


Database management

system

Communication in Between

program

files

SQL

database

storage

Data

SQLSTATE


How to Use SQLSTATE

• An easy way is to treat it as a string• Special considerations for host languages,

e.g., C/C++– A string in C/C++ needs an end-of-string

character ‘\0’– Declare char SQLSTATE[6];– Set SQLSTATE[5]=‘\0’;– Use strcmp in comparison


Storing Results into Variables

• If an SQL statement returns only one tuple, the attribute values can be stored into variables in the host language

• Use keyword INTO in the SELECT clauseEXEC SQL SELECT SUM(assets)

INTO :total_assetsFROM branchWHERE branch_city = ‘Brooklyn’;


Handling a Set of Tuples

• Generally, an SQL statement may return a set of tuples

• Cursor – Conceptually, put all tuples returned by a query

into a file– Fetch the tuples from the virtual file– After use, the file should be closed and released


Example

• From within a host language, find the names and cities of customers with more than the variable amount dollars in some accountEXEC SQL

declare c cursor for select customer_name, customer_cityfrom depositor, customer, accountwhere depositor.customer_name = customer.customer_name

and depositor account_number = account.account_numberand account.balance > :amount

END_EXEC

• Declared, but not materialized yet


Executing a Cursor

• The open statement causes the query to be evaluated

EXEC SQL open c END_EXEC• The fetch statement causes the values of

one tuple in the query result to be placed on the host language variablesEXEC SQL fetch c into :cn, :cc END_EXEC– Can be called repeatedly to get all tuples


Executing a Cursor (2)

• SQLSTATE set to ‘02000’ to indicate no more data is available

• The close statement causes the database system to delete the temporary relation that holds the result of the query

EXEC SQL close c END_EXEC• Details vary with language

– The Java embedding defines Java iterators to step through result tuples


Updates Through Cursors

• To update tuples fetched by cursor, declare the cursor for update

declare c cursor forselect * from accountwhere branch_name = ‘Perryridge’

for update• To update tuple at the current balance of cursor c

update accountset balance = balance + 100where current of c


Concurrent Changes

• Two users may change tuples in a database concurrently– Process 1: remove account of balance < 1000– Process 2: increase the balance by 10%– Whether an account of balance 950 would be removed

depends on which process touches the tuple first• Keyword INSENSITIVE makes a cursor not

affected by other concurrent updatesEXEC SQL DECLARE accountCursor INSENSITIVE

CURSOR FORSELECT balance FROM account;

– More about concurrency control in CMPT 454


READ ONY Cursors

• If a cursor is used only for read operations, it can be concurrent with any other processesEXEC SQL DECLARE readAccCursor CURSOR

READ_ONLY FOR SELECT balance FROM account;

• Different database systems may have slightly different syntax


Scrolling Cursors• Random access versus serial access

– Serial access: fetch the next one, cannot jump to other tuples

– Random access: fetch any tuple in a cursor• Scrolling cursor: a cursor for random access

EXEC SQL DECLARE accCursor SCROLL CURSOR FOR account;

• Options in FECTCH– NEXT or PRIOR– FIRST or LAST– RELATIVE k (RELATIVE -1 = PRIOR)– ABSOLUTE k (ABSOLUTE -1 = LAST)


Dynamic SQL

• Allow programs to construct and submit SQL queries at run time

char * sqlprog = “update account set balance = balance * 1.05

where account_number = ?”EXEC SQL prepare dynprog from :sqlprog;char account [10] = “A-101”;EXEC SQL execute dynprog using :account;

• “?” is a place holder for a value that is provided when the SQL program is executed


Summary

• SQL is good for accessing relational data, but not for other general problem solving tasks– Embedding SQL into a host general

programming language is useful in practice• Embedded SQL• Cursors


To-Do-List

• Understand how to embed SQL into C#• Understand cursors in SQL Server 2005

Documents

Basics of Database Tuning - Computing · PDF fileCMPT 354: Database I -- Embedded SQL 2 SQL Is Not for All • SQL is capable in manipulating relational data • SQL is not good for