Check MySQL database for unique value over many tables - mysql

Question

I'm looking for a way to easily check each table of a MySQL database and make sure that a certain field contains one value only. I have tables named Authors, Titles, Places, etc.
Each table contains a field called xuser and it needs to ask "does the field xuser contain the value xy in all records of all tables".
Can someone push me in the right direction how to do this with a SQL query if this is possible?
Thanks for reading, regards
Nico

Answer 1

I've created stored procedure which checks all table for provided db:
DELIMITER $$
DROP PROCEDURE IF EXISTS `UTL_CHECK_BACKUP_FOR_USER` $$
CREATE PROCEDURE `UTL_CHECK_BACKUP_FOR_USER`(
IN i_database_name VARCHAR(255),
IN i_user_column_name VARCHAR(255),
IN i_user_column_value VARCHAR(255),
OUT o_result TINYINT(1)
)
BEGIN
DECLARE v_table_name VARCHAR(255);
DECLARE v_last_row_fetched TINYINT(3) DEFAULT 0;
DECLARE tables_cursor CURSOR FOR
SELECT table_name
FROM information_schema.tables
WHERE table_schema = i_database_name
;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET v_last_row_fetched = 1;
SET v_last_row_fetched = 0;
OPEN tables_cursor;
SET #query =
CONCAT(
'SELECT SUM(IF(user_column=''',
i_user_column_value,
''', 1, -1)) = 1 INTO #o_result FROM ( SELECT ''test'' AS user_column FROM information_schema.tables WHERE 1<>1 '
)
;
table_loop: LOOP
FETCH tables_cursor INTO v_table_name;
IF (v_last_row_fetched = 1) THEN
LEAVE table_loop;
END IF;
SET #query =
CONCAT(
#query,
' UNION SELECT DISTINCT(',
i_user_column_name,
') AS user_column FROM ',
v_table_name
)
;
END LOOP table_loop;
CLOSE tables_cursor;
SET v_last_row_fetched=0;
SET #query =
CONCAT(
#query,
' ) all_xusers;'
)
;
PREPARE stmt FROM #query;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SET o_result = COALESCE(#o_result, 0);
END $$
DELIMITER ;
Just deploy this stored procedure to database.
And then it could be executed in the following way:
-- db_name, user_column_name, user_column_value, result
call UTL_CHECK_BACKUP_FOR_USER('test', 'xuser', 'xxx', #result);
select #result;

Answer 2

To get the rows from all three tables where xuser has the same value in all three tables you could use:
SELECT *
FROM authors a
JOIN titles t
ON t.xuser = a.xuser
JOIN places p
ON p.xuser = t.xuser
If you want to look at a specific xuser value you could add the following WHERE clause:
WHERE a.xuser = 'xy'

Answer 3

The first thing comes to my mind:
select sum(if(xuser='xxx', 1, -1)) = 1
from (
select distinct(xuser) from authors
union
select distinct(xuser) from titles
union
select distinct(xuser) from places
) all_xusers;
This will return 1 (true) if all tables contains records belonging ONLY to 'xxx' user. Otherwise (if there is no 'xxx' records or there is some other user records) it will return 0 (false).