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Add Count The Number Of Items In An Array as a Postgres TIL

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README.md
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@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
_1574 TILs and counting..._
_1575 TILs and counting..._
See some of the other learning resources I work on:
- [Ruby Operator Lookup](https://www.visualmode.dev/ruby-operators)
@@ -780,6 +780,7 @@ See some of the other learning resources I work on:
- [Convert A String To A Timestamp](postgres/convert-a-string-to-a-timestamp.md)
- [Count How Many Records There Are Of Each Type](postgres/count-how-many-records-there-are-of-each-type.md)
- [Count Records By Type](postgres/count-records-by-type.md)
- [Count The Number Of Items In An Array](postgres/count-the-number-of-items-in-an-array.md)
- [Count The Number Of Trues In An Aggregate Query](postgres/count-the-number-of-trues-in-an-aggregate-query.md)
- [Create A Cluster In A Specific Data Directory](postgres/create-a-cluster-in-a-specific-data-directory.md)
- [Create A Composite Primary Key](postgres/create-a-composite-primary-key.md)

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postgres/count-the-number-of-items-in-an-array.md Normal file
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# Count The Number Of Items In An Array
There are two ways to count the number of items in an array with PostgreSQL.
The one that might jump out at you or show up at the top of search results is
[`array_length`](https://www.postgresql.org/docs/current/functions-array.html).
```sql
> select array_length(array[1,2,3], 1);
+--------------+
| array_length |
|--------------|
| 3 |
+--------------+
> select array_length(array[[1,2], [3,4]], 2);
+--------------+
| array_length |
|--------------|
| 2 |
+--------------+
```
This requires specifying the dimension at which you want to check the length.
The first example, checking the 1st dimension of a one-dimensional array, seems
like the more common and useful scenario. In the second example, we are
checking the 2nd dimension.
The other way we can determine the number of items in an array is with the
[`cardinality`](https://www.postgresql.org/docs/current/functions-array.html)
function.
> Returns the total number of elements in the array, or 0 if the array is
> empty.
```sql
> select cardinality(array[1,2,3]);
+-------------+
| cardinality |
|-------------|
| 3 |
+-------------+
> select cardinality(array[[1,2], [3,4]]);
+-------------+
| cardinality |
|-------------|
| 4 |
+-------------+
```
This behaves the same as `array_length` for a one-dimensional array and doesn't
require a second argument. Where it gets more interesting is with
multi-dimensional arrays. It returns the total number of elements in the
arrayregardless of the nesting.
[source](https://mattrighetti.com/2025/01/20/you-dont-need-sql-builders)