Resolve an error whenever multiple rows are returned in MySQL Benchmark?

You will get an error whenever you return multiple rows in the benchmark. Return a scalar value or single row instead of multiple rows. The syntax is as follows −

SELECT yourColumnName FROM yourTableName WHERE yourCondition.

To understand the above syntax, let us create a table. The query to create a table is as follows −

mysql> create table UserDemo
   -> (
   -> UserId int,
   -> UserName varchar(20),
   -> RegisteredCourse varchar(10)
   -> );
Query OK, 0 rows affected (0.67 sec)

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

mysql> insert into UserDemo values(1,'John','Java');
Query OK, 1 row affected (0.16 sec)
mysql> insert into UserDemo values(2,'Larry','C');
Query OK, 1 row affected (0.21 sec)
mysql> insert into UserDemo values(3,'Carol','C++');
Query OK, 1 row affected (0.11 sec)
mysql> insert into UserDemo values(4,'Mike','C#');
Query OK, 1 row affected (0.17 sec)

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

mysql> select *from UserDemo;

The following is the output −

+--------+----------+------------------+
| UserId | UserName | RegisteredCourse |
+--------+----------+------------------+
|      1 | John     | Java             |
|      2 | Larry    | C                |
|      3 | Carol    | C++              |
|      4 | Mike     | C#               |
+--------+----------+------------------+
4 rows in set (0.00 sec)

Your benchmark error is as follows −

mysql> select benchmark(1000,(select RegisteredCourse from UserDemo));
ERROR 1242 (21000): Subquery returns more than 1 row

Since the subquery returned more than one row, therefore you need to return a single row. The query is as follows −

mysql> select RegisteredCourse from UserDemo where UserId=1;

The following is the output −

+------------------+
| RegisteredCourse |
+------------------+
| Java             |
+------------------+
1 row in set (0.00 sec)