docs/sql-ref-syntax-qry-select-unnest.md

---
layout: global
title: UNNEST Clause
displayTitle: UNNEST Clause
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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 `NULL`s. 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

```sql
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

```sql
-- 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|
+---+----+
```

### Related Statements

* [SELECT](sql-ref-syntax-qry-select.html)
* [LATERAL VIEW Clause](sql-ref-syntax-qry-select-lateral-view.html)
* [Table-valued Function](sql-ref-syntax-qry-select-tvf.html)