Add Set Value On Null JSON Column as a MySQL 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).
 
-_1428 TILs and counting..._
+_1429 TILs and counting..._
 
 ---
 
@@ -611,6 +611,7 @@ _1428 TILs and counting..._
 - [List Databases And Tables](mysql/list-databases-and-tables.md)
 - [Run Statements In A Transaction](mysql/run-statements-in-a-transaction.md)
 - [Select Rows After An Offset](mysql/select-rows-after-an-offset.md)
+- [Set Value On Null JSON Column](mysql/set-value-on-null-json-column.md)
 - [Show Create Statement For A Table](mysql/show-create-statement-for-a-table.md)
 - [Show Tables That Match A Pattern](mysql/show-tables-that-match-a-pattern.md)
 - [Show Indexes For A Table](mysql/show-indexes-for-a-table.md)

mysql/set-value-on-null-json-column.md

@@ -0,0 +1,44 @@
+# Set Value On Null JSON Column
+
+To set a key-value pair on a JSON field, you can reach for
+[MySQL's `json_set`](https://dev.mysql.com/doc/refman/8.0/en/json-modification-functions.html#function_json-set)
+or one of [the other JSON setter
+functions](https://dev.mysql.com/doc/refman/8.0/en/json-modification-functions.html).
+
+However, if the JSON field you are updating is `null`, you might get an
+unexpected result.
+
+```sql
+> update User
+    set metadata = json_set(metadata, '$.discord_id', 'discord_123')
+    where id = 123;
+
+Query OK, 0 rows affected (0.00 sec)
+Rows matched: 1  Changed: 0  Warnings: 0
+```
+
+We can see that the `where` clause matched on a single row as expected, but
+right above that it says _0 rows affected_.
+
+What happened?
+
+The `json_set` function is not able to set a key-value pair on `null`. It needs
+a JSON object to work on.
+
+There are a number of ways to get around this. I find that
+[`coalesce`](https://dev.mysql.com/doc/refman/8.0/en/comparison-operators.html#function_coalesce)
+is a natural way to handle this. If `metadata` happens to be `null`, then we
+_coalesce_ it to `'{}'` (an empty object).
+
+```sql
+> update User
+    set metadata = json_set(coalesce(metadata, '{}'), '$.discord_id', 'discord_123')
+    where id = 123;
+
+Query OK, 1 row affected (0.02 sec)
+Rows matched: 1  Changed: 1  Warnings: 0
+```
+
+It updates as expected. That same statement will work on a row where `metadata`
+already contains a JSON object since the `coalesce` will resolve to that
+instead of the empty object.