How to update empty string to NULL in MySQL?

For this, use LENGTH(), since if the length is 0 that would mean the string is empty. After finding, you can set it to NULL using the SET clause in the UPDATE command. Let us first create a table −

mysql> create table DemoTable
(
   Name varchar(50)
);
Query OK, 0 rows affected (0.68 sec)

Insert some records in the table using insert command −

mysql> insert into DemoTable values('Chris');
Query OK, 1 row affected (0.18 sec)
mysql> insert into DemoTable values('');
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable values('David');
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable values('');
Query OK, 1 row affected (0.15 sec)

Display all records from the table using select statement −

mysql> select *from DemoTable;

This will produce the following output −

+-------+
| Name  |
+-------+
| Chris |
|       |
| David |
|       |
+-------+
4 rows in set (0.00 sec)

Following is the query to update empty string to NULL −

mysql> update DemoTable set Name=NULL where length(Name)=0;
Query OK, 2 rows affected (0.12 sec)
Rows matched: 2 Changed: 2 Warnings: 0

Let us check the table records once again −

mysql> select *from DemoTable;

This will produce the following output −

+-------+
| Name  |
+-------+
| Chris |
| NULL  |
| David |
| NULL  |
+-------+
4 rows in set (0.00 sec)