Implement CASE statement with WHEN clause to check for the existence of a record in MySQL

Let us first create a table −

mysql> create table DemoTable
(
   UserId int,
   UserName varchar(100)
);
Query OK, 0 rows affected (0.57 sec)

Insert some records in the table using insert command −

mysql> insert into DemoTable values(100,'Chris');
Query OK, 1 row affected (0.13 sec)
mysql> insert into DemoTable values(101,'Robert');
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable values(102,'David');
Query OK, 1 row affected (0.12 sec)
mysql> insert into DemoTable values(103,'Mike');
Query OK, 1 row affected (0.11 sec)

Display all records from the table using select statement −

mysql> select *from DemoTable;

This will produce the following output −

+--------+----------+
| UserId | UserName |
+--------+----------+
|    100 | Chris    |
|    101 | Robert   |
|    102 | David    |
|    103 | Mike     |
+--------+----------+
4 rows in set (0.00 sec)

Following is the query for CASE WHEN −

mysql> select case
   when exists(select * from DemoTable where UserId=102)
      then 'Present'
   else 'Not Present'
      end as Result;

This will produce the following output −

+---------+
| Result  |
+---------+
| Present |
+---------+
1 row in set (0.02 sec)