left join在實際應用中並不是查出資料都是左邊的全部資料，為什麼呢？看看自己的解決方法吧

SELECT
       T.xx,
       T1.xx,
       ...
  FROM xxx T
  LEFT JOIN xxx T1
    ON T.ID = T1.ID
 WHERE T.xx = xx
   and T1.xx= xx

1.在left join後增加WHERE條件中關於T1的過濾條件

SELECT
       T.xx,
       T1.xx,
       ...
  FROM xxx T
  LEFT JOIN (SELECT xx,xx2,... FROM xxx WHERE xx= x) 
 
T1
    ON T.ID = T1.ID
 WHERE T.xx = xx

SELECT
       T.xx,
       T1.xx,
       ...
  FROM xxx T
  LEFT JOIN xxx T1
    ON T.ID = T1.ID AND T.xx = xx AND T1.xx = xx