Add Swich Non-Castable Column Type With Using Clause as a Postgres til

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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-_1003 TILs and counting..._
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@@ -560,6 +560,7 @@ _1003 TILs and counting..._
 - [Special Math Operators](postgres/special-math-operators.md)
 - [Storing Emails With citext](postgres/storing-emails-with-citext.md)
 - [String Contains Another String](postgres/string-contains-another-string.md)
+- [Switch Non-Castable Column Type With Using Clause](postgres/switch-non-castable-column-type-with-using-clause.md)
 - [Temporarily Disable Triggers](postgres/temporarily-disable-triggers.md)
 - [Temporary Tables](postgres/temporary-tables.md)
 - [Terminating A Connection](postgres/terminating-a-connection.md)

postgres/switch-non-castable-column-type-with-using-clause.md

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+# Switch Non-Castable Column Type With Using Clause
+
+With certain data types, such as from `int` to `bigint` or `timestamptz` to
+`timestamp`, there is an automatic type casting that can take place with
+existing data. This means Postgres knows how to handle a data type change
+like:
+
+```sql
+alter table users
+  alter column id
+  set data type bigint;
+```
+
+With other data types, such as `int` to `uuid`, there is no way for Postgres to
+know how to automatically cast it. To change the data type of a column in a
+scenario like this, you have to tell Postgres how to handle the conversion with
+a `using` clause.
+
+```sql
+alter table users
+  alter column id
+  set data type uuid using (gen_random_uuid());
+```
+
+In this instance, the `using` clause tells Postgres to ignore the existing
+integer `id` value and use the `gen_random_uuid()` function to generate a UUID
+value to take its place.
+
+The `using` clause can also reference the existing column value as part of its
+type cast.
+
+See the [alter table
+documentation](https://www.postgresql.org/docs/current/sql-altertable.html) for
+more details on this.