add unnest

Author: kidd

2023-12-xx-rank-sensitivity.md 2023-12-06-rank-sensitivity.md

@@ -0,0 +1,30 @@
+---
+title: Unnest arrays from multiple rows
+categories: [sql]
+---
+
+Once you start moving off of the classic relational model in a sql
+database and start using arrays you keep needing to put an array back
+into a table when further dealing with the values.
+
+A basic case is when using an array of values from your programming language into a sql query.
+
+That's how I usually do it:
+
+```sql
+with users as (select id::uuid from unnest(?) as id)
+select id from users;
+```
+
+Another common usecase for arrays is when aggregating values in a
+group by. There you got to different groups bucketed in different
+arrays, and you may want to get a subset of them in table form:
+
+
+```sql
+with users as (select status, count(*), array_agg(id) as ids from users group by status)
+  select unnest(ids) from (select ids from users where status in ('active', 'inactive')) as t;
+```
+
+It's a nice pattern to keep in mind, while reminding that functions
+can usually run in the "from" side, but also in the "select" side.