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5.3.2 Column Labels

The entries in the select list can be assigned names for subsequent processing, such as for use in an ORDER BY clause or for display by the client application. For example:

SELECT a AS value, b + c AS sum FROM ...

If no output column name is specified using AS, the system assigns a default column name. For simple column references, this is the name of the referenced column. For function calls, this is the name of the function. For complex expressions, the system will generate a generic name.

The AS keyword is optional, but only if the new column name does not match any PostgreSQL keyword (see appendix C SQL Key Words). To avoid an accidental match to a keyword, you can double-quote the column name. For example, VALUE is a keyword, so this does not work:

SELECT a value, b + c AS sum FROM ...

but this does:

SELECT a "value", b + c AS sum FROM ...

For protection against possible future keyword additions, it is recommended that you always either write AS or double-quote the output column name.

Note: The naming of output columns here is different from that done in the FROM clause (see section 5.2.1.2 Table and Column Aliases). It is possible to rename the same column twice, but the name assigned in the select list is the one that will be passed on.