Question

I have a table with the following structure:

(table_name, column_name) 

and for each row in this table I need to query the column_name in the table_name and do a COUNT(column_name) GROUP BY column_name of the values in there.

Currently I do

SELECT * FROM this table 
/*and then*/ foreach row: do another query with: 
  SELECT column_name, COUNT(column_name) GROUP BY column_name

Is there any way I can do this in a single query?
Something like

SELECT column_name, COUNT(column_name) 
GROUP BY column_name 
FOREACH(SELECT table_name, column_name FROM my_initial_table)

I know the last query is not valid, it’s just an example for what I am looking to to achieve if it’s possible.

LE:

The table that tells me where to look has 2 varchar columns

Ex:

|++++++++++++++++++++++++++++++++
| table_name | column_name      |
|+++++++++++++++++++++++++++++++|
| logsa      | foo              |
|===============================|
| logsa      | bar              |
|===============================|
| testx      | baz              |
|===============================|

This tells me that now i have too look in columns foo and bar of table logsa and column baz of table testx

Every column in every table has VARCHAR as datattype and i just need to count those that are the same. that’s way i was doing

SELECT column_name, COUNT(column_name) 
    GROUP BY column_name

Answer 1

If you are working in MySql, you can’t directly use parametrized column names.
There is an indirect way of doing this using stored procedures and prepared statements.

some sloppy first-draft code…
notice the difference between backticks ` and quotes ‘

  CREATE PROCEDURE CountTables()
  BEGIN
      DECLARE done TINYINT DEFAULT 0;
      DECLARE table_name varchar(30), colunn_name varchar(30);
      DECLARE cur1 CURSOR FOR SELECT
          table_name, column_name
       FROM ColumnTable;

      DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;
      CREATE TEMPORARY TABLE t1( table_name varchar(30), column_name varchar(30), count int);

      OPEN cur1;
      START TRANSACTION;

    read_loop: LOOP
        FETCH FROM cur1 INTO table_name, column_name;
        IF done THEN LEAVE read_loop; END IF;

        SET insert_sql = CONCAT(  "INSERT INTO `t1`(`table_name`, `column_name`, `count`) SELECT ",
                                   "'", table_name, "', '", column_name, "', count(`", column_name, "`)",
                                  " FROM `",    table_name, "`"  
                               );

        PREPARE insert_stmt FROM insert_sql;
        EXECUTE insert_stmt;
        DEALLOCATE PREPARE insert_stmt;

      END LOOP;
      COMMIT;
      SELECT * FROM t1 GROUP BY column_name;
      DROP TEMPORARY TABLE t1;

    END;

Oh ya, don’t forget to call the procedure:

CALL CountTables();