rendered paste bodymysql> select null as prueba;
+--------+
| prueba |
+--------+
| NULL |
+--------+
1 row in set (0.00 sec)
mysql> select coalesce(false,"vacio") as prueba;
+--------+
| prueba |
+--------+
| 0 |
+--------+
1 row in set (0.00 sec)
mysql> select coalesce(5.5,"vacio") as prueba;
+--------+
| prueba |
+--------+
| 5.5 |
+--------+
1 row in set (0.00 sec)
mysql> select coalesce("string","vacio") as prueba;
+--------+
| prueba |
+--------+
| string |
+--------+
1 row in set (0.00 sec)
@mysql> select coalesce(null,"vacio") as prueba;
+--------+
| prueba |
+--------+
| vacio |
+--------+
1 row in set (0.00 sec)