MySQL从多个表中选择一条记录

Question

I have 400+ tables and each has a column timestamp. I'd like to pull the first row/record for timestamp for each of these tables. I don't want to run 400+ queries, what would the best way to do this be?

Answer 1

You can run a UNION query like this:

SELECT Id, yourColumn, timestamp FROM Table001 ORDER BY timestamp DESC LIMIT 1
UNION ALL
SELECT Id, yourColumn, timestamp FROM Table002 ORDER BY timestamp DESC LIMIT 1
UNION ALL
SELECT Id, yourColumn, timestamp FROM Table003 ORDER BY timestamp DESC LIMIT 1
ORDER BY timestamp 

Use a loop to create this statement:

SET @resultQuery = NULL;
SELECT
  GROUP_CONCAT(
    DISTINCT
    CONCAT('SELECT * FROM ', table_name, ' ORDER BY timestamp DESC LIMIT 1 ')
    SEPARATOR '\r\nUNION\r\n'
  )
INTO
  @resultQuery
FROM
  information_schema.COLUMNS
WHERE
  table_schema = '???' AND column_name LIKE '%timestamp';

SELECT @resultQuery;

Answer 2

The need for the first record from 400+ tables smells like poor design, but if they have the same schema, you can use UNION ALL :

SELECT * FROM foo ORDER BY timestamp ASC LIMIT 1
UNION ALL
SELECT * FROM bar ORDER BY timestamp ASC LIMIT 1
UNION ALL
SELECT * FROM baz ORDER BY timestamp ASC LIMIT 1
-- etc...