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

References

1. Fehily, Chris (2005). SQL: Visual Quickstart Guide (2 ed.). Peachpit Press. pp. 439–440, 480. ISBN 978-0-321-33417-6. SQL Keywords [...] The appendix lists the SQL:2003 standard's reserved and non-reserved keywords. [...] EXISTS [...]
2. 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.