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

README.md

@@ -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)

postgres/count-the-number-of-items-in-an-array.md

@@ -0,0 +1,56 @@
+# 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)