MySQL update column to NULL for blank values

For this, you can use IF() along with UPDATE command. Let us first create a table −

mysql> create table DemoTable1601
   -> (
   -> FirstName varchar(20) ,
   -> LastName varchar(20)
   -> );
Query OK, 0 rows affected (0.53 sec)

Insert some records in the table using insert command −

mysql> insert into DemoTable1601 values('John','Doe');
Query OK, 1 row affected (0.10 sec)
mysql> insert into DemoTable1601 values('Adam','');
Query OK, 1 row affected (0.16 sec)
mysql> insert into DemoTable1601 values('David','Miller');
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable1601 values('Chris','');
Query OK, 1 row affected (0.11 sec)

Display all records from the table using select statement −

mysql> select * from DemoTable1601;

This will produce the following output −

+-----------+----------+
| FirstName | LastName |
+-----------+----------+
| John      |      Doe |
| Adam      |          |
| David     |   Miller |
| Chris     |          |
+-----------+----------+
4 rows in set (0.00 sec)

Following is the query to update column to null or its value −

mysql> update DemoTable1601 set LastName=if(LastName='',NULL,LastName);
Query OK, 2 rows affected (0.22 sec)
Rows matched: 4  Changed: 2 Warnings: 0

Let us check the table records once again −

mysql> select * from DemoTable1601;

This will produce the following output −

+-----------+----------+
| FirstName | LastName |
+-----------+----------+
| John      |      Doe |
| Adam      |     NULL |
| David     |   Miller |
| Chris     |     NULL |
+-----------+----------+
4 rows in set (0.00 sec)