Condition (SQL)

A relational database management system uses SQL conditions or expressions in <tt>WHERE</tt> clauses and in <tt>HAVING</tt> clauses to <tt>SELECT</tt> 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 <code>EXISTS</code>

to determine whether rows exist in a subquery result.

Examples

To <tt>SELECT</tt> one row of data from a table called tab with a primary key column (pk) set to 100 &mdash; 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 &mdash; use the condition dk = 100 and the condition having count(*) > 1:

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

References

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