多表關聯查詢

兩表關聯查詢的sql語句，咱們會發現限制條件不一樣是，查詢出來的結果是不同的。

1.下面我們使用左鏈接查詢：

select a.id,a.articlename,a.articledes,a.listimg,a.author,a.click,b.userid from yxdj_post as a left join yxdj_postfavorite as b on a.id=b.articlecode and b.userid='$userid' where a.`status`=1 order by $sort $order limit $offset,$pagesize

當咱們設置$userid變量爲1041288時，咱們將會看到以下結果：

2.去掉左連接（left）查詢:

select a.id,a.articlename,a.articledes,a.listimg,a.author,a.click,b.userid from yxdj_post as a join yxdj_postfavorite as b on a.id=b.articlecode and b.userid='$userid' where a.`status`=1 order by $sort $order limit $offset,$pagesize

select a.id,a.articlename,a.articledes,a.listimg,a.author,a.click,b.userid from yxdj_post as a left join yxdj_postfavorite as b on a.id=b.articlecode  where a.`status`=1 and b.userid='$userid' order by $sort $order limit $offset,$pagesize

3.若是咱們把限制條件更換一下位置同時也設置$userid變量爲1041288時會發現：