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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, SNOWFLAKE, SQLSERVER, 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 3.19, see #10141), or translate your own on our website
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