SQL/Lesson 4/Slide 1 of 45 Using Subqueries and Managing Databases Objectives In this lesson, you will learn to: Use subqueries Use subqueries with

SQL/Lesson 4/Slide 1 of 45

Using Subqueries and Managing Databases

Objectives

In this lesson, you will learn to:

Use subqueries

Use subqueries with the IN clause

Use subqueries with the EXISTS clause

Use nested subqueries

Use correlated subqueries

Use the SELECT INTO statement

Use the UNION operator

View, rename, and delete databases



Subqueries

A subquery can be defined as a SELECT query that returns a single value

Subqueries are nested within a SELECT, INSERT, UPDATE, or DELETE statement

Subqueries can be used to retrieve data from multiple tables and can be used as an alternative to a join

Subqueries can also be used inside the WHERE or HAVING clause of the SELECT, INSERT, UPDATE, and DELETE statements



4.D.1 Using one Query in Another

List the contract recruiters who live in the same city as the external candidate Barbara Johnson.



Task List

Create a format for the query output

Identify the components of the query

Execute the query

Verify that the query output is as per the required results




Result

The required output from the query is the names of the contract recruiters who reside in the same city as 'Barbara Johnson'

The required data is present in the ContractRecruiter and ExternalCandidate tables



Draft the query

Result

The required information is available in the ExternalCandidate and ContractRecruiter tables

Therefore, the query using the SELECT statement should be:

SELECT cName

FROM ContractRecruiter

WHERE cCity = (SELECT cCity

FROM ExternalCandidate

WHERE vFirstName = 'Barbara'

AND vLastName = 'Johnson')



Execute the query

Action:

In the Query Analyzer window, type the query

Execute the query




Action:

Check whether:

The required rows are displayed



More on Subqueries

Subqueries with IN

A subquery introduced with IN returns zero or more values

Example

SELECT Au_Id

FROM TitleAuthor

WHERE Title_Id IN

(SELECT Title_Id FROM Sales)



More on Subqueries (Contd.)

Subqueries with EXISTS

A subquery, when used with the EXISTS clause, always returns data in terms of a TRUE or FALSE value

Example

SELECT Pub_Name

FROM Publishers

WHERE EXISTS (SELECT * FROM Titles

WHERE Type = 'business')




Subqueries with Aggregate Functions

Aggregate functions can also be used in subqueries

Example

SELECT Title

FROM Titles

WHERE Advance > (SELECT AVG(Advance)

FROM Titles

WHERE Type = 'business')




Subqueries Restrictions SQL Server restricts the use of certain methods and

techniques, and forces the implementation of certain standards while using subqueries. The restrictions imposed are:

The column list of the SELECT statement of a subquery introduced with a comparison operator can include only one column

The column used in the WHERE clause of the outer query should be compatible with the column used in the SELECT list of the inner query




The ORDER BY clause and the GROUP BY clause cannot be used in the inner query when =, !=, <, <=, >, or >= are used in the main query, as the inner query may return more than one value that cannot be handled by the outer query




Nested Subqueries

A subquery can itself contain one or more subqueries

Example

SELECT 'Author Name' = SUBSTRING

(Au_Fname, 1, 1) + '. '+ Au_Lname

FROM Authors

WHERE Au_Id IN (SELECT Au_Id

FROM TitleAuthor

WHERE Title_Id =(SELECT

Title_Id FROM Titles

WHERE Title = 'Net Etiquette'))




Correlated Subqueries

Can be defined as queries that depend on the outer query for its evaluation

Example

SELECT Title, Type, Advance

FROM Titles t1

WHERE t1.Advance > (SELECT AVG(t2.Advance)

FROM Titles t2 WHERE

t1.Type = t2.Type)




Queries With Modified Comparison Operators

SQL Server provides the ALL and ANY keywords that can be used to modify the existing comparison operator



4.D.2 Extracting Data Into Another Table

To carry out an analysis of the profile of candidates who have applied for recruitment in May 2001, you need to copy their details into a new table.



Task List

Identify the output requirements of the query

Draft the query

Execute the query




Identify the output requirements of the query

Result:

The required output from the query is the transfer of data from the ExternalCandidate table to a temporary table called tempExternalCandidate



Draft the query

SELECT INTO Statement

A SELECT statement with the INTO clause is used to store the result set in a new table without a data definition process. The SELECT INTO statement creates a new table.

Syntax SELECT columns_listINTO new_table_nameFROM table_name1, table_name2,………, table_name nWHERE condition1, condition2,………., condition n



Draft the query (Contd.)

Example

SELECT Title_Id, Title

INTO NewTitles

FROM Titles

WHERE Price > $15

Result:

The required information is available in the ExternalCandidate table





SELECT * INTO tempExternalCandidate

FROM ExternalCandidate

WHERE DATEPART(mm,dDateOfApplication)= 5

AND DATEPART(yyyy,dDateOfApplication)= 2001



Execute the query

Action


Execute the query




Action:

Check whether:

The target table has all data from the source table

Just a Minute…

Write a query to copy all the contents of InternalCandidate to a table called backupInternalCandidate.



4.D.3 Combining Data From Two Tables

A list of contract recruiters and recruitment agencies along with their phone numbers is required.



Task List


Draft the query

Execute the query





Result:

The required output from the query is a single list of names and phone numbers of contract recruiters and recruitment agencies



Draft the query

UNION Operator

Is used to combine the result set of two or more queries

Syntax SELECT column_list [INTO new_table_name][FROM clause] [WHERE clause] [GROUP BY clause][HAVING clause] [UNION [ALL] SELECT column_list [FROM clause] [WHERE clause] [GROUP BY clause][HAVING clause]...] [ORDER BY clause] [COMPUTE clause]




Result

The required information is available in the ContractRecruiter and RecruitmentAgencies tables


SELECT cName,cPhone FROM ContractRecruiterUNION SELECT cName, cPhone

FROM RecruitmentAgencies



Execute the query

Action


Execute the query




Action:

Check whether:

All the required columns are displayed

All rows from both tables are displayed as one list

Just a Minute…

Display the list of college names, newspaper names, and their addresses in the following format:

Name Address



Databases

A database is a collection of tables and objects such as views, indexes, stored procedures, and triggers

System Databases master Database-records all the server-specific

configuration information, including authorized users, databases, system configuration settings, and remote servers

tempdb Database-is a temporary database that is used as an interim storage area

model Database-acts as a template or a prototype for the new databases

msdb Database-supports SQL Server Agent



Databases (Contd.)

System Tables

Are a set of tables that are used by SQL server to store information about configuration, security, and object information

SQL Server manages each database with the help of the system tables, which contain all the system information

Just a Minute…

What is the purpose of model database in SQL Server?



Databases (Contd.)

Files: The three types of files that a database has are:

Primary

Secondary

Transaction Log

Filegroup: Is a collection of files

A file or filegroup cannot be used by more than one database

A file can be a member of only one filegroup



Viewing a Database

The information regarding the database such as owner, size, date of creation, and status can be viewed using the following:

sp_helpdb database_name



Renaming a Database

The name of a database can be changed using the sp_renamedb command. The database should not be in use when it is being renamed, and it should be set to the single-user mode Syntaxsp_renamedb 'old_name', 'new_name'



Deleting a Database

The DROP DATABASE statement is used to delete a database

SyntaxDROP DATABASE database_name

Just a Minute…

List the three types of operating system files that store the data and objects of SQL Server database.



Summary

In this lesson, you learned that:

Subqueries are nested within a SELECT, INSERT, UPDATE, or DELETE statement

A subquery can be used inside the WHERE or HAVING clauses of the outer SELECT, INSERT, UPDATE, or DELETE statements

The subquery introduced with IN or NOT IN returns zero or more values

The subquery used with the EXISTS clause returns data in terms of TRUE or FALSE



Summary (Contd.)

A subquery can contain one or more subqueries. There is no restriction on the number of subqueries one can include with the SELECT, INSERT, UPDATE, or DELETE statements

A correlated subquery can be defined as a query that depends on the outer query for its evaluation

A SELECT statement with an INTO clause can be used to store the result set in a new table without any data definition process

The UNION operator is used to combine the result set of two or more queries into one



Summary (Contd.)

By default the result set of a UNION operator removes the duplicate rows from the queries combined, unless an ALL clause is specified with the UNION operator

A database consists of a collection of tables with data and other objects such as views, indexes, stored procedures, and triggers

SQL Server has the following system databases:

master

tempdb

model

msdb



Summary (Contd.) SQL Server stores its configuration, security, and object

information in tables called system tables

A database consists of the following types of files:

Primary data file

Secondary data file

Transaction log file

A filegroup is a collection of files. Filegroups allow files to be grouped together

A database comprises of a primary filegroup and any user-defined filegroup(s)



Summary (Contd.)

Creation of a database involves determining the name of the database, the size of the database, and the files used to store data in the database

sp_helpdb is used to view the information regarding a database

sp_renamedb is used to rename a database

The DROP DATABASE statement is used to delete a database

Documents

SQL/Lesson 4/Slide 1 of 45 Using Subqueries and Managing Databases Objectives In this lesson, you will learn to: *Use subqueries * Use subqueries with

SQL/Lesson 4/Slide 1 of 45 Using Subqueries and Managing Databases Objectives In this lesson, you will learn to: Use subqueries Use subqueries with