layout: global title: UNNEST Clause displayTitle: UNNEST Clause license: | Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to You under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

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Description

The UNNEST clause expands one or more arrays into a table that can be referenced in the FROM clause, producing one row per array element. It is the ANSI SQL collection derived table and is a standard alternative to explode / LATERAL VIEW.

When several arrays are supplied, they are expanded in parallel: the number of output rows equals the length of the longest array, and shorter arrays are padded with NULLs. A NULL array is treated as an empty array and contributes no elements.

To reference a column of another FROM item (a correlated array), use UNNEST on the right-hand side of a LATERAL join. A LEFT JOIN LATERAL ... ON true preserves outer rows whose array is empty or NULL.

UNNEST is a non-reserved keyword, so it can still be used as a regular table or column name. However, when it appears unquoted at the start of a FROM relation followed by (, it is parsed as the UNNEST clause described here rather than as a call to a table-valued function named unnest. To invoke such a function instead, quote the name: `unnest`(...).

Syntax

UNNEST ( expression [ , ... ] ) [ WITH ORDINALITY ] [ table_alias ]

Parameters

  • expression

    One or more array-typed expressions to expand. Each array contributes one output column, holding its element as-is (an array of structs is not expanded into one column per field).

  • WITH ORDINALITY

    Appends a trailing 1-based BIGINT column giving the position of each element.

  • table_alias

    Specifies a temporary name with an optional column name list.

    Syntax: [ AS ] table_name [ ( column_name [ , ... ] ) ]

Examples

-- single array
SELECT * FROM UNNEST(array(10, 20, 30));
+---+
|col|
+---+
| 10|
| 20|
| 30|
+---+

-- WITH ORDINALITY and a table alias
SELECT * FROM UNNEST(array('a', 'b')) WITH ORDINALITY AS t(value, pos);
+-----+---+
|value|pos|
+-----+---+
|    a|  1|
|    b|  2|
+-----+---+

-- multiple arrays are expanded in parallel and padded with NULLs
SELECT * FROM UNNEST(array(1, 2), array(10, 20, 30)) AS t(a, b);
+----+---+
|   a|  b|
+----+---+
|   1| 10|
|   2| 20|
|NULL| 30|
+----+---+

-- correlated UNNEST over a table column, via LATERAL
SELECT id, elem
FROM VALUES (1, array(10, 20)), (2, array(30)) AS data(id, arr),
     LATERAL UNNEST(arr) AS t(elem);
+---+----+
| id|elem|
+---+----+
|  1|  10|
|  1|  20|
|  2|  30|
+---+----+

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