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Derived column lists

Applies to ✅ Open Source Edition   ✅ Express Edition   ✅ Professional Edition   ✅ Enterprise Edition

The SQL standard specifies how a table can be renamed / aliased in one go along with its columns. It references the term "derived column list" for the following syntax:

SELECT t.a, t.b
FROM (
  SELECT 1, 2
) t(a, b)

This feature is useful in various use-cases where column names are not known in advance (but the table's degree is!). An example for this are unnested tables, or the VALUES() table constructor:

-- Unnested tables
SELECT t.a, t.b
FROM unnest(my_table_function()) t(a, b)

-- VALUES() constructor
SELECT t.a, t.b
FROM VALUES(1, 2),(3, 4) t(a, b)

Only few databases really support such a syntax, but fortunately, jOOQ can emulate it easily using UNION ALL and an empty dummy record specifying the new column names. The two statements are equivalent:

-- Using derived column lists
SELECT t.a, t.b
FROM (
  SELECT 1, 2
) t(a, b)

-- Using UNION ALL and a dummy record
SELECT t.a, t.b
FROM (
  SELECT null a, null b FROM DUAL WHERE 1 = 0
  UNION ALL
  SELECT 1, 2 FROM DUAL
) t

In jOOQ, you would simply specify a varargs list of column aliases as such:

// Unnested tables
create.select().from(unnest(myTableFunction()).as("t", "a", "b")).fetch();

// VALUES() constructor
create.select().from(values(
  row(1, 2),
  row(3, 4)
).as("t", "a", "b"))
.fetch();

Dialect support

This example using jOOQ:

selectFrom(values(row(1, 2)).as("t", "a", "b"))

Translates to the following dialect specific expressions:

Access

SELECT t.a, t.b
FROM (
  SELECT
    1 a,
    2 b
  FROM (
    SELECT count(*) dual
    FROM MSysResources
  ) AS dual
) t

ASE, Redshift, SQLDataWarehouse, Vertica

SELECT t.a, t.b
FROM (
  SELECT 1, 2
) t (a, b)

Aurora MySQL, MemSQL

SELECT t.a, t.b
FROM (
  SELECT
    1 a,
    2 b
  FROM DUAL
) t

Aurora Postgres, CockroachDB, DB2, Derby, DuckDB, Exasol, H2, HSQLDB, Oracle, Postgres, SQLServer, Snowflake, Trino, YugabyteDB

SELECT t.a, t.b
FROM (
  VALUES (1, 2)
) t (a, b)

BigQuery

SELECT t.a, t.b
FROM (
  SELECT
    null a,
    null b
  FROM UNNEST([STRUCT(1 AS dual)]) AS dual
  WHERE FALSE
  UNION ALL
  SELECT *
  FROM UNNEST ([ STRUCT (1, 2)]) t
) t

Firebird

SELECT t.a, t.b
FROM (
  SELECT 1, 2
  FROM RDB$DATABASE
) t (a, b)

Hana

SELECT t.a, t.b
FROM (
  SELECT
    1 a,
    2 b
  FROM SYS.DUMMY
) t

Informix

SELECT t.a, t.b
FROM (
  TABLE (MULTISET { ROW (1, 2)})
) t (a, b)

MariaDB

SELECT t.a, t.b
FROM (
  SELECT
    1 a,
    2 b
) t

MySQL

SELECT t.a, t.b
FROM (
  VALUES ROW (1, 2)
) t (a, b)

SQLite

SELECT t.a, t.b
FROM (
  SELECT
    null a,
    null b
  WHERE 1 = 0
  UNION ALL
  SELECT *
  FROM (
    VALUES (1, 2)
  ) t
) t

Sybase

SELECT t.a, t.b
FROM (
  SELECT 1, 2
  FROM SYS.DUMMY
) t (a, b)

Teradata

SELECT t.a, t.b
FROM (
  SELECT 1, 2
  FROM (
    SELECT 1 AS "dual"
  ) AS "dual"
) t (a, b)

(These are currently generated with jOOQ 3.20, see #10141), or translate your own on our website

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