Untitled

mysql> 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)