我有一個MySQL JOIN包括4個表:差異加盟連鎖和SELECT子查詢LEFT JOIN
直接鏈接
SELECT col1, col2, col3... col12 FROM
(((tbl1 LEFT JOIN tbl2...) LEFT JOIN tbl3 ...) LEFT JOIN tbl4);
子選擇
(SELECT col10 .. col12 FROM
(SELECT col7 .. col9 FROM
(SELECT col1, ... col6 FROM tbl1
LEFT JOIN tbl2) AS J1
LEFT JOIN tbl3) AS J2
LEFT JOIN tbl4...)
是這兩種方法之間存在效率差異?我的直覺是,子選擇使用SELECT ... WHERE子句放棄不必要的行和列,並使JOIN更快,內存更少。有什麼建議?其他數據庫如何?