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]

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 column dk — use the condition having count(*) > 1:

SELECT dk FROM tab GROUP BY dk HAVING count(*) > 1

↑ Fehily, Chris (2005). SQL: Visual Quickstart Guide (2 ed.). Peachpit Press. pp. 439–440, 480. ISBN 978-0-321-33417-6. https://archive.org/details/sql0000fehi/page/439. "SQL Keywords [...] The appendix lists the 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. https://archive.org/details/sql0000fehi/page/278. "EXISTS and NOT EXISTS [...] look for the existence or nonexistence of rows in a subquery result."

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