Querying multiple databases at once

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');