A relational database management system uses SQL conditions or expressions in WHERE clauses and in HAVING clauses to SELECT subsets of data.
Contents
Types of condition
EXISTS
to determine whether rows exist in a subquery result.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:
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:
References
Condition (SQL) Wikipedia(Text) CC BY-SA