Английская Википедия:Condition (SQL)
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A relational database management system uses SQL conditions or expressions in [[Where (SQL)|Шаблон:Mono]] clauses and in [[Having (SQL)|Шаблон:Mono]] clauses to Шаблон:Mono 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 Шаблон:Mono 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
References
