This is typically used to assign names to column that don’t have a use-assigned name—such as columns produced by values or unnest:
SELECT b, a
FROM (VALUES (1, 'a')
) t(a, b)
The SQL standard requires all columns to be renamed.
Variants
Deviations
Some systems allow renaming fewer columns as the table has so that only the first few columns are renamed. Some tested systems does not enforce uniqueness of names.
E051-09, “Rename columns in the FROM clause”, is an mandatory feature of ISO/IEC 9075-2:2023. The functionality first appeared in SQL:92 (intermediate).0 It became mandatory in 1999.
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About the Author
Markus Winand provides insights into SQL and shows how different systems support it at modern-sql.com. Previously he made use-the-index-luke.com, which is still actively maintained. Markus can be hired as trainer, speaker and consultant via winand.at.
