Update a column A if null, else update column B, else if both columns are not null do nothing with MySQL

For this, use IF() with IS NULL property. Let us first create a table −

mysql> create table DemoTable1976
   (
   FirstName varchar(20),
   LastName varchar(20)
   );
Query OK, 0 rows affected (0.00 sec)

Insert some records in the table using insert command −

mysql> insert into DemoTable1976 values('John','Doe');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1976 values('John',NULL);
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1976 values(NULL,'Miller');
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1976 values('Chris','Brown');
Query OK, 1 row affected (0.00 sec)

Display all records from the table using select statement −

mysql> select * from DemoTable1976;

This will produce the following output −

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

Here is the query to update a column if null else update anther column, else if both columns are not null do nothing −

mysql> update DemoTable1976
   set FirstName=if(FirstName IS NULL,'David',FirstName),
   LastName=if(LastName IS NULL,'Brown',LastName);
Query OK, 2 rows affected (0.00 sec)
Rows matched: 4  Changed: 2 Warnings: 0

Let us check the table records once again −

mysql> select * from DemoTable1976;

This will produce the following output −

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