Subquery: Inner Queries

A second SELECT statement (Subquery (subquery)) can be included in a SELECT statement. A value or set of values is generated by the Subquery as part of the main statement.

Prerequisites

You can use the demo data for the SQL tutorial. Start the Database Studio as database administrator MONA with the password RED and log on to demo database DEMODB: Logging On to a Database.

Activities

You can use Database Studio to enter and execute SQL statements. More information: Working with SQL Statements: Overview

Subquery

SELECT name, zip

FROM hotel.customer

WHERE zip = (SELECT MAX(zip) FROM hotel.customer)

Displaying the customer with the greatest zip code

Result

`NAME`	`ZIP`
`Doe`	`97213`

See also: SQL Reference Manual, Comparison Predicate (comparison_predicate)

SELECT hno, name, zip

FROM hotel.hotel

WHERE zip IN(SELECT zip FROM hotel.customer WHERE title = 'Mrs')

Displaying the hotels located in cities/places in which female customers also live

Result

`HNO`	`NAME`	`ZIP`
`10`	`Congress`	`20005`

See also: SQL Reference Manual, IN Predicate (in_predicate)

SELECT hno, name

FROM hotel.hotel

WHERE name = ANY(SELECT name FROM hotel.city)

Displaying hotels that have the same name as other cities in the city table. The inner query determines the list of city names with which the hotel names are compared.

Result

`HNO`	`NAME`
`20`	`Long Island`
`120`	`Long Beach`

SELECT cno, name

FROM hotel.customer

WHERE cno = ANY(SELECT cno FROM hotel.reservation WHERE arrival > '2005-01-01')

Displaying the customers that have made all reservations as of a specified date

Result

`CNO`	`NAME`
`3600`	`Smith`
`3900`	`Howe`

Use ANY if the Subquery returns more than one value and you want to take this into consideration in the WHERE clause condition that usually requires exactly one value.

See also: SQL Reference Manual, Quantified Predicate (quantified_predicate)

Correlated Subquery

You can use Correlated Subqueries to formulate conditions for selecting rows; these conditions are to apply to groups of rows only and not to all rows in a table.

Subqueries are only evaluated once. Correlated Subqueries are evaluated for each row in the external table, in the case of compound Subqueries from the inside out.

SELECT hno, type, price

FROM hotel.room room_table

WHERE price =

(SELECT MAX(price) FROM hotel.room WHERE type = room_table.type)

Displaying the hotels (in the form of the hotel number) whose rooms in the individual types (single, double, or suite) have the highest price

Since in this example, the same table is addressed in the external SELECT statement as in the Subquery, you have to specify a reference name (room_table). A row from the result of the external SELECT statement is linked (correlated) to a value in the Subquery.

Explanation

SELECT hno, type, price FROM hotel.room room_table Searches for the hotel number, room type and price in the room table and renames the table room_table.
WHERE price= Contains the row in which the price is the same as the result of the following Subquery:
(SELECT MAX(price) FROM hotel.room (Start the Subquery) Searches for the maximum price in the room table
WHERE type = room_table.type) The room types here are to be the same as the room types in the rows found above

Result

`HNO`	`TYPE`	`PRICE`
`130`	`double`	`270`
`130`	`single`	`160`
`130`	`suite`	`700`

SELECT cno, title, name

FROM hotel.customer

WHERE EXISTS

(SELECT * FROM hotel.reservation

WHERE hotel.customer.cno = hotel.reservation.cno)

Displaying only those customers from the customer table for which one or more reservations exist

Result

`CNO`	`TITLE`	`NAME`
`3000`	`Mrs`	`Porter`
`3100`	`Mr`	`Brown`
`3200`	`Company`	`Datasoft`
`3600`	`Mrs`	`Smith`
`3900`	`Mr`	`Howe`
`4100`	`Mrs`	`Baker`
`4300`	`Company`	`TOOLware`
`4400`	`Mr`	`Jenkins`

Use EXISTS if you only want the Subquery to find out whether a row exists that fulfils a specific condition.

See also: SQL Reference Manual, EXISTS Predicate (exists_predicate)

More Information

More examples for Data Query