參考:https://bbs.csdn.net/topics/330032307
數據庫里面有一個字典表,這張表里面有id段和對應的名字字段。
在另外一個記錄的表里面有對應的上述字典表的id,而且有多個字段都含有該id字段。需要將其查出來,並且用名字。
1、第一種寫法,不使用 left join
SELECT a.id AS '編號', 'STORY' AS '類型' , a.title AS '標題',b.name AS '所屬項目' , c.realname AS '創建者', a.estimate AS '計划時間', d.realname AS '責任研發', a.resolvedDate AS '實際解決時間',a.closedDate AS '實際關閉時間',a.STATUS FROM zt_story a , zt_project b, zt_user c, zt_user d WHERE a.project=b.id AND a.openedby=c.account AND a.assignedTo=d.account AND a.id>=8650 AND a.id<= 8692 ORDER BY a.id
查詢出的結果是嚴格的比照,為35條,比期望的43條少了8條。因為字典表內容缺少一些。
SELECT a.id AS '編號', 'BUG' AS '類型' , a.title AS '標題',b.name AS '所屬項目' , c.realname AS '創建者', a.estimateDoneDate AS '計划時間', d.realname AS '責任研發', a.resolvedDate AS '實際解決時間',a.closedDate AS '實際關閉時間',a.STATUS FROM zt_bug a , zt_project b, zt_user c, zt_user d WHERE a.project=b.id AND a.openedby=c.account AND a.assignedTo=d.account AND a.id>=28095 ORDER BY a.id
查出29條記錄。
2、使用Left join方法,可以全部查出來,43條,一些字典表里沒有的數據對應字段為空
SELECT a.id AS '編號', 'STORY' AS '類型' , a.title AS '標題',b.name AS '所屬項目' , c.realname AS '創建者', a.estimate AS '計划時間', d.realname AS '責任研發', a.resolvedDate AS '實際解決時間',a.closedDate AS '實際關閉時間',a.STATUS FROM zt_story a LEFT JOIN zt_project b ON a.project=b.id LEFT JOIN zt_user c ON a.openedby=c.account LEFT JOIN zt_user d ON a.assignedTo=d.account WHERE a.id>=8650 AND a.id<= 8692 ORDER BY a.id
上述語句完全將期望的43條記錄全部查出,只是有8條記錄的部分字段是空的。
SELECT a.id AS '編號', 'BUG' AS '類型' , a.title AS '標題',b.name AS '所屬項目' , c.realname AS '創建者', a.estimateDoneDate AS '計划時間', d.realname AS '責任研發', a.resolvedDate AS '實際解決時間',a.closedDate AS '實際關閉時間',a.STATUS FROM zt_bug a LEFT JOIN zt_project b ON a.project=b.id LEFT JOIN zt_user c ON a.openedby=c.account LEFT JOIN zt_user d ON a.assignedTo=d.account WHERE a.id>=28095 ORDER BY a.id
該表對應的字典表數據均存在,所以上述語句執行后查詢到的結果也是29條。
做個標記。
總結:
根據自己的期望,如果需要嚴格比照,不要用 LEFT JOIN。
如果需要顯示全部記錄,使用 LEFT JOIN
https://blog.csdn.net/flexitime/article/details/2380531 Delphi中寫復雜SQL語句的簡便方式