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Queries are a method of requesting information from a database. A specialised programming language called Structured Query Language (SQL) is often used to create queries that retrieve information from a database.

SQL allows a user to SELECT fields FROM a table WHERE certain criteria are met.

For example, from the table below, SELECT 'Firstname','Lastname' FROM 'Customer ID' WHERE 'Town = Belfast':

This would return: Bob Buchannon.

The criteria for a SELECT query can contain:

• boolean operators (AND, OR, NOT)
• comparison operators (<, >, =, <=, >=)

The results of a query will always be returned as a table.

Boolean operators

• AND - Returns a record if both the first AND second condition is true
• OR - Returns a record if either the first OR second condition is true
• NOT - Only returns a record if the condition is NOT true

Comparison operators

• > - Is one value greater than another?
• < - Is one value less than another?
• = - Is one value equal to another?
• <= - Is one value less than OR equal to another?
• >= - Is one value greater than OR equal to another?