Check a column for unique value in MySQL

You can use subquery for this. Let us first create a demo table

mysql> create table uniqueBothColumnValueSameDemo
   -> (
   -> UserId int,
   -> UserValue int
   -> );
Query OK, 0 rows affected (0.64 sec)

Insert some records in the table using insert command. The query is as follows −

mysql> insert into uniqueBothColumnValueSameDemo values(10,20);
Query OK, 1 row affected (0.21 sec)
mysql> insert into uniqueBothColumnValueSameDemo values(10,20);
Query OK, 1 row affected (0.09 sec)
mysql> insert into uniqueBothColumnValueSameDemo values(20,30);
Query OK, 1 row affected (0.10 sec)
mysql> insert into uniqueBothColumnValueSameDemo values(20,40);
Query OK, 1 row affected (0.11 sec)
mysql> insert into uniqueBothColumnValueSameDemo values(50,10);
Query OK, 1 row affected (0.10 sec)
mysql> insert into uniqueBothColumnValueSameDemo values(50,10);
Query OK, 1 row affected (0.07 sec)
mysql> insert into uniqueBothColumnValueSameDemo values(60,30);
Query OK, 1 row affected (0.07 sec)
mysql> insert into uniqueBothColumnValueSameDemo values(60,30);
Query OK, 1 row affected (0.08 sec)
mysql> insert into uniqueBothColumnValueSameDemo values(60,50);
Query OK, 1 row affected (0.12 sec)

Display all records from the table using select statement. The query is as follows −

mysql> select *from uniqueBothColumnValueSameDemo;

The output is as follows

+--------+-----------+
| UserId | UserValue |
+--------+-----------+
|     10 |        20 |
|     10 |        20 |
|     20 |        30 |
|     20 |        40 |
|     50 |        10 |
|     50 |        10 |
|     60 |        30 |
|     60 |        30 |
|     60 |        50 |
+--------+-----------+
9 rows in set (0.00 sec)

Here is the query to check column for unique value

mysql> SELECT DISTINCT UserId, UserValue
   -> FROM uniqueBothColumnValueSameDemo tbl1
   -> WHERE (SELECT count(DISTINCT UserValue)
   -> FROM uniqueBothColumnValueSameDemo tbl2
   -> WHERE tbl2.UserId = tbl1.UserId) = 1;

The output is as follows

+--------+-----------+
| UserId | UserValue |
+--------+-----------+
|     10 |        20 |
|     50 |        10 |
+--------+-----------+
2 rows in set (0.00 sec)