Condition (SQL)

A relational database management system uses SQL conditions or expressions in WHERE clauses and in HAVING clauses to SELECT subsets of data.

Types of condition

Many conditions compare values for (for example) equality, inequality or similarity.

The EXISTS condition uses the SQL standard keyword EXISTS^[1] to determine whether rows exist in a subquery result.^[2]

Examples

To SELECT one row of data from a table called tab with a primary key column (pk) set to 100 — use the condition pk = 100:

SELECT * FROM tab WHERE pk = 100

To identify whether a table tab has rows of data with a duplicated key column dk set to 100 — use the condition dk = 100 and the condition having count(*) > 1:

SELECT * FROM tab WHERE dk = 100 having count(*) > 1

SQL

Versions	SQL-86 SQL-89 SQL-92 SQL:1999 SQL:2003 SQL:2006 SQL:2008 SQL:2011

Keywords	As Case Create Delete From Having Insert Join Merge Null Order by Prepare Select Truncate Union Update Where With

Related	Edgar Codd Relational database

ISO/IEC SQL parts	Framework Foundation Call-Level Interface Persistent Stored Modules Management of External Data Object Language Bindings Information and Definition Schemas SQL Routines and Types for the Java Programming Language XML-Related Specifications

References

↑ Fehily, Chris (2005). SQL: Visual Quickstart Guide (2 ed.). Peachpit Press. p. 480. ISBN 978-0-321-33417-6. SQL Keywords [...] The appendix lists the SQL:2003 standard's reserved and non-reserved keywords. [...] EXISTS [...]
↑ Fehily, Chris (2005). SQL: Visual Quickstart Guide (2 ed.). Peachpit Press. p. 278. ISBN 978-0-321-33417-6. EXISTS and NOT EXISTS [...] look for the existence or nonexistence of rows in a subquery result.