MySQL concat() to create column names to be used in a query?

MySQLMySQLi Database

To create column names to be used in a query, you need to use a user-defined variable with the set command. The syntax is as follows −

SET @anyVariableName :=
(
   SELECT CONCAT
   (
      "SELECT",
   GROUP_CONCAT(CONCAT("\n 1 as ", COLUMN_NAME) SEPARATOR ','), "\n FROM DUAL")
   FROM INFORMATION_SCHEMA_COLUMNS
   WHERE TABLE_NAME= ‘yourTableName’
);

Now prepare the statement using the PREPARE command. The syntax is as follows −

PREPARE anyVariableName from @anyVariableName;

Execute statement using EXECUTE command. The syntax is as follows −

EXECUTE anyVariableName;

Deallocate the prepared statement using DEALLOCATE command. The syntax is as follows −

DEALLOCATE PREPARE anyVariableName;

We will now create a table with two columns and column will be used in the concat query. The query to create a table is as follows −

mysql> create table ConcatenationWithUserDefinedVariable
   -> (
   -> Id int NOT NULL AUTO_INCREMENT,
   -> User_Id int,
   -> PRIMARY KEY(Id)
   -> );
Query OK, 0 rows affected (1.14 sec)

Use the above syntax for column names to be used in the query. The query is as follows −

mysql> set @q := (
   -> select concat(
   -> "select",
   -> group_concat(concat("\n 1 as ", column_name) separator ','),
   -> "\nfrom dual")
   -> from information_schema.columns
   -> where table_name = 'ConcatenationWithUserDefinedVariable');
Query OK, 0 rows affected (0.01 sec)

The query to prepare the above user-defined variable is as follows −

mysql> prepare stmt from @q;
Query OK, 0 rows affected (0.00 sec)
Statement prepared

Now execute the above-prepared statement. The query is as follows −

mysql> execute stmt;

The following is the output −

+----+---------+
| Id | User_Id |
+----+---------+
|  1 |       1 |
+----+---------+
1 row in set (0.00 sec)

In the end, you need to deallocate the prepared statement. The query is as follows −

mysql> deallocate prepare stmt;
Query OK, 0 rows affected (0.00 sec)
raja
Published on 26-Feb-2019 10:35:58
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