SQL SELECT

Database notes

aggregate functions

SQL functions that return a summary of data. These include: average (AVG), min, max, sum, and count.

NULL

The absence of a value/value is unknown. Not a blank! Can cause issues if used in conditional evaluation (e.g., false and null is null, not false).

GROUP BY

A SQL clause used to create frequency distributions when combined with any of the aggregate functions in a SELECT command.

HAVING

A clause applied to the output of a GROUP BY operation to restrict the selected rows.

nested query (subquery or inner query)

A query within another query. In a SELECT command, a nested query can be used in the SELECT, FROM, or WHERE clauses.

WITH

A SQL clause that provides a way of defining a temporary relation whose definition is available only to the query in which the WITH clause occurs.