Count number of occurrences in SQL - mysql

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();

Answer 2

A subquery should be able to help you here. This is an example (untested) which should work with some refinement. I am going to call your table Schema for the purposes of my example, and in the subquery I am going to alias it as ColumnCount to make the code (hopefully) more readable.
SELECT Schema.table_name, Schema.column_name,
(SELECT COUNT(*) FROM Schema ColumnCount
WHERE Schema.column_name = ColumnCount.column_name) AS ColumnUsageCount
FROM Schema

Answer 3

No need to do a subselect, just do this, if there's only one table involved.
SELECT max(table_name) as table_name, column_name, COUNT(*) as occurrence
FROM initial_table
GROUP BY column_name
ORDER BY column_name
The Max(table_name) substitutes for the non-existent function Whatever(table_name).
In MySQL you can also use group_concat(table_name) as table_names. Try it out!
The ORDER BY is optional (and in MySQL not needed) but can be handy if you want it sorted by column name as well.
If you want to list the unique occurrences of the combi table_name + column_name do:
SELECT table_name, column_name, COUNT(*) as occurrence
FROM initial_table
GROUP BY table_name, column_name
ORDER BY table_name, column_name

Answer 4

in PHP
$result=mysql_query("SELECT column_name FROM $table_name");
$row_count = mysql_num_rows($result);
I'm not too familiar with mysql, but i think its along those lines