PostgreSQL 9.1.24 Documentation
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# 9.17. Array Functions and Operators

Table 9-41 shows the operators available for array types.

Table 9-41. Array Operators

Operator | Description | Example | Result
--- | --- | --- | ---
`=` | equal | `ARRAY[1.1,2.1,3.1]::int[] = ARRAY[1,2,3]` | `t`
`<>` | not equal | `ARRAY[1,2,3] <> ARRAY[1,2,4]` | `t`
`<` | less than | `ARRAY[1,2,3] < ARRAY[1,2,4]` | `t`
`>` | greater than | `ARRAY[1,4,3] > ARRAY[1,2,4]` | `t`
`<=` | less than or equal | `ARRAY[1,2,3] <= ARRAY[1,2,3]` | `t`
`>=` | greater than or equal | `ARRAY[1,4,3] >= ARRAY[1,4,3]` | `t`
`@>` | contains | `ARRAY[1,4,3] @> ARRAY[3,1]` | `t`
`<@` | is contained by | `ARRAY[2,7] <@ ARRAY[1,7,4,2,6]` | `t`
`&&` | overlap (have elements in common) | `ARRAY[1,4,3] && ARRAY[2,1]` | `t`
`||` | array-to-array concatenation | `ARRAY[1,2,3] || ARRAY[4,5,6]` | `{1,2,3,4,5,6}`
`||` | array-to-array concatenation | `ARRAY[1,2,3] || ARRAY[[4,5,6],[7,8,9]]` | `{{1,2,3},{4,5,6},{7,8,9}}`
`||` | element-to-array concatenation | `3 || ARRAY[4,5,6]` | `{3,4,5,6}`
`||` | array-to-element concatenation | `ARRAY[4,5,6] || 7` | `{4,5,6,7}`

Array comparisons compare the array contents element-by-element, using the default B-tree comparison function for the element data type. In multidimensional arrays the elements are visited in row-major order (last subscript varies most rapidly). If the contents of two arrays are equal but the dimensionality is different, the first difference in the dimensionality information determines the sort order. (This is a change from versions of PostgreSQL prior to 8.2: older versions would claim that two arrays with the same contents were equal, even if the number of dimensions or subscript ranges were different.)

See Section 8.14 for more details about array operator behavior.

Table 9-42 shows the functions available for use with array types. See Section 8.14 for more information and examples of the use of these functions.

Table 9-42. Array Functions

Function | Return Type | Description | Example | Result
--- | --- | --- | --- | ---
``array_append`(`anyarray`, `anyelement`)` | `anyarray` | append an element to the end of an array | `array_append(ARRAY[1,2], 3)` | `{1,2,3}`
``array_cat`(`anyarray`, `anyarray`)` | `anyarray` | concatenate two arrays | `array_cat(ARRAY[1,2,3], ARRAY[4,5])` | `{1,2,3,4,5}`
``array_ndims`(`anyarray`)` | `int` | returns the number of dimensions of the array | `array_ndims(ARRAY[[1,2,3], [4,5,6]])` | `2`
``array_dims`(`anyarray`)` | `text` | returns a text representation of array's dimensions | `array_dims(ARRAY[[1,2,3], [4,5,6]])` | `[1:2][1:3]`
``array_fill`(`anyelement`, `int[]`, [, `int[]`])` | `anyarray` | returns an array initialized with supplied value and dimensions, optionally with lower bounds other than 1 | `array_fill(7, ARRAY[3], ARRAY[2])` | `[2:4]={7,7,7}`
``array_length`(`anyarray`, `int`)` | `int` | returns the length of the requested array dimension | `array_length(array[1,2,3], 1)` | `3`
``array_lower`(`anyarray`, `int`)` | `int` | returns lower bound of the requested array dimension | `array_lower('[0:2]={1,2,3}'::int[], 1)` | `0`
``array_prepend`(`anyelement`, `anyarray`)` | `anyarray` | append an element to the beginning of an array | `array_prepend(1, ARRAY[2,3])` | `{1,2,3}`
``array_to_string`(`anyarray`, `text` [, `text`])` | `text` | concatenates array elements using supplied delimiter and optional null string | `array_to_string(ARRAY[1, 2, 3, NULL, 5], ',', '*')` | `1,2,3,*,5`
``array_upper`(`anyarray`, `int`)` | `int` | returns upper bound of the requested array dimension | `array_upper(ARRAY[1,8,3,7], 1)` | `4`
``string_to_array`(`text`, `text` [, `text`])` | `text[]` | splits string into array elements using supplied delimiter and optional null string | `string_to_array('xx~^~yy~^~zz', '~^~', 'yy')` | `{xx,NULL,zz}`
``unnest`(`anyarray`)` | `setof anyelement` | expand an array to a set of rows | `unnest(ARRAY[1,2])` |
    1
    2
(2 rows)

In `string_to_array`, if the delimiter parameter is NULL, each character in the input string will become a separate element in the resulting array. If the delimiter is an empty string, then the entire input string is returned as a one-element array. Otherwise the input string is split at each occurrence of the delimiter string.

In `string_to_array`, if the null-string parameter is omitted or NULL, none of the substrings of the input will be replaced by NULL. In `array_to_string`, if the null-string parameter is omitted or NULL, any null elements in the array are simply skipped and not represented in the output string.

> Note: There are two differences in the behavior of `string_to_array` from pre-9.1 versions of PostgreSQL. First, it will return an empty (zero-element) array rather than NULL when the input string is of zero length. Second, if the delimiter string is NULL, the function splits the input into individual characters, rather than returning NULL as before.

See also Section 9.18 about the aggregate function `array_agg` for use with arrays.

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