Querying multiple databases at once - mysql

Question

I have WordPress instances with each in its own database. For an update I need to query all active plugins, that are stored in a table 'wp_options' and accessible via
WHERE option_name='active_plugins'
How can I access all active plugin settings (spread over multiple databases) and output them in one single SQL result? I know the database.tablename syntax, but how do I go on from there with the above Where statement?
A request in a single database would look like this:
SELECT option_value
FROM `database`.`wp_options`
WHERE option_name="active_plugins"

Answer 1

SELECT option_value
FROM `database1`.`wp_options`
WHERE option_name="active_plugins"
UNION
SELECT option_value
FROM `database2`.`wp_options`
WHERE option_name="active_plugins"

Answer 2

The solution by Pentium10 is good but its drawback is that you have to extend the query for every schema to be included. The below solution uses a prepared statement to produce a result set for all schemas on your MySQL server which have the wp_options table. This should be more convenient for you.
DROP PROCEDURE IF EXISTS `MultipleSchemaQuery`;
DELIMITER $$
CREATE PROCEDURE `MultipleSchemaQuery`()
BEGIN
declare scName varchar(250);
declare q varchar(2000);
DROP TABLE IF EXISTS ResultSet;
create temporary table ResultSet (
option_value varchar(200)
);
DROP TABLE IF EXISTS MySchemaNames;
create temporary table MySchemaNames (
schemaName varchar(250)
);
insert into MySchemaNames
SELECT distinct
TABLE_SCHEMA as SchemaName
FROM
`information_schema`.`TABLES`
where
TABLE_NAME = 'wp_options';
label1:
LOOP
set scName = (select schemaName from MySchemaNames limit 1);
set #q = concat('select option_value from ', scName, '.wp_options where option_name=\'active_plugins\'');
PREPARE stmt1 FROM #q;
EXECUTE stmt1;
DEALLOCATE PREPARE stmt1;
delete from MySchemaNames where schemaName = scName;
IF ((select count(*) from MySchemaNames) > 0) THEN
ITERATE label1;
END IF;
LEAVE label1;
END LOOP label1;
SELECT * FROM ResultSet;
DROP TABLE IF EXISTS MySchemaNames;
DROP TABLE IF EXISTS ResultSet;
END
$$
DELIMITER ;
CALL MultipleSchemaQuery();

Answer 3

Gruber's answer works great, but it has a syntax error --- there's a spurious comma at the end of line 10. Here is the code, with syntax error fixed:
DELIMITER $$
CREATE PROCEDURE `MultipleSchemaQuery`()
BEGIN
declare scName varchar(250);
declare q varchar(2000);
DROP TABLE IF EXISTS ResultSet;
create temporary table ResultSet (
option_value varchar(200)
);
DROP TABLE IF EXISTS MySchemaNames;
create temporary table MySchemaNames (
schemaName varchar(250)
);
insert into MySchemaNames
SELECT distinct
TABLE_SCHEMA as SchemaName
FROM
`information_schema`.`TABLES`
where
TABLE_NAME = 'wp_options';
label1:
LOOP
set scName = (select schemaName from MySchemaNames limit 1);
set #q = concat('select option_value from ', scName, '.wp_options where option_name=\'active_plugins\'');
PREPARE stmt1 FROM #q;
EXECUTE stmt1;
DEALLOCATE PREPARE stmt1;
delete from MySchemaNames where schemaName = scName;
IF ((select count(*) from MySchemaNames) > 0) THEN
ITERATE label1;
END IF;
LEAVE label1;
END LOOP label1;
SELECT * FROM ResultSet;
DROP TABLE IF EXISTS MySchemaNames;
DROP TABLE IF EXISTS ResultSet;
END
$$

Answer 4

Yet another example of querying multiple databases using procedure, cursor, union all and prepared statement. Does not require drop and delete permissions:
USE `my_db`;
DROP PROCEDURE IF EXISTS `CountAll`;
DELIMITER $$
CREATE PROCEDURE `CountAll`(IN tableName VARCHAR(255))
BEGIN
DECLARE db_name VARCHAR(250);
DECLARE exit_loop BOOLEAN;
DECLARE union_query TEXT DEFAULT '';
DECLARE my_databases CURSOR FOR
SELECT DISTINCT `table_schema`
FROM `information_schema`.`tables`
WHERE
`table_schema` LIKE 'myprefix\_%' AND
`table_name` = tableName;
DECLARE CONTINUE HANDLER
FOR NOT FOUND SET exit_loop = TRUE;
OPEN my_databases;
get_database: LOOP
FETCH my_databases INTO db_name;
IF exit_loop THEN
-- remove trailing UNION ALL statement
SET union_query = TRIM(TRAILING ' UNION ALL ' FROM union_query);
LEAVE get_database;
END IF;
SET union_query = concat(union_query, 'SELECT COUNT(*) AS qty FROM ',
db_name, '.', tableName, ' UNION ALL ');
END LOOP get_database;
CLOSE my_databases;
SET #final_query = concat('SELECT SUM(qty) FROM (', union_query,
') AS total;');
PREPARE stmt1 FROM #final_query;
EXECUTE stmt1;
DEALLOCATE PREPARE stmt1;
END$$
DELIMITER ;
CALL CountAll('wp_options');