外部連接和自聯接 inner join(等值連接) 只返回兩個表中聯結字段相等的行 left join(左聯接) 返回包括左表中的所有記錄和右表中聯結字段相等的記錄 right join(右聯接) 返回包括右表中的所有記錄和左表中聯結字段相等的記錄 on 指定表間聯結字段及其關系的等號 "=" 表達式, 返回 true 或 false. 當表達式返回 true 時, 則查詢中包含該記錄. ! 外部連接只能操作已存在于數據庫中的數據
7.5.2
程序代碼 Update (ctarticle AS a LEFT JOIN ctclass AS c ON a.classid = c.classid) LEFT JOIN cttag AS b ON a.articleid = b.articleid SET tag=tag+' ', b.articleid=a.articleid, b.classid=a.classid, b.nclassid=a.nclassid Where a.classid=23 AND a.nclassid=0 AND tagid is not null
7.5.1
程序代碼 Update (ctarticle AS a LEFT JOIN (ctnclass AS c LEFT JOIN ctclass AS d ON c.classid = d.classid) ON a.nclassid = c.nclassid AND a.classid = c.classid) LEFT JOIN cttag AS b ON a.articleid = b.articleid SET tag=d.class+' '+c.nclass, b.articleid=a.articleid, b.classid=a.classid, b.nclassid=a.nclassid Where a.classid=23 AND a.nclassid=197;
7.5 更新操作
74.5 左連接中數據的篩選
程序代碼 Insert INTO cttag(articleid,classid,nclassid) Select a.articleid,a.classid,a.nclassid from ctarticle a left join cttag b on a.articleid=b.articleid where b.articleid is null
程序代碼 Select a.*, b.*, c.*, d.* FROM cttag as d left join ((ctarticle AS a LEFT JOIN ctclass AS b ON a.classid=b.classid) LEFT JOIN ctnclass AS c ON a.nclassid=c.nclassid) on d.articleid=a.articleid;
7.4.4 顯示文章表中的全部, 調用類別表中的欄目
程序代碼 select a.*, b.*, c.* from (ctarticle a left join ctclass b on a.classid=b.classid) left join ctnclass c on a.nclassid=c.nclassid
//作用, 有時在文章表中包含了在個別類別表中沒有的數據, 用這個語法可以讀出文章表的全部數據 //a 為 文章表, b 為主類別, c 為子類別
7.4.3 同上例, 選擇追加數據時加上空格
程序代碼 Insert INTO cttag(articleid,classid,nclassid,tag) Select a.articleid,a.classid,a.nclassid,d.class+' '+c.nclass FROM (ctarticle AS a left join (ctnclass c left join ctclass d on c.classid=d.classid) on a.classid=c.classid and a.nclassid=c.nclassid) LEFT JOIN cttag AS b ON a.articleid = b.articleid where a.classid=4 and a.nclassid=154;
7.4.2 連接N個表, 并追加數據到其中一個表, N=4
程序代碼 Insert INTO cttag(articleid,classid,nclassid,tag) Select a.articleid,a.classid,a.nclassid,d.class+c.nclass FROM (ctarticle AS a left join (ctnclass c left join ctclass d on c.classid=d.classid) on a.classid=c.classid and a.nclassid=c.nclassid) LEFT JOIN cttag AS b ON a.articleid = b.articleid where a.classid=1 and a.nclassid=1;
程序代碼 Insert INTO cttag(articleid,classid,nclassid) Select a.articleid,a.classid,a.nclassid FROM ctarticle AS a LEFT JOIN cttag AS b ON a.articleid = b.articleid where a.classid=1 and a.nclassid=1;
程序代碼 select a.*, b.* from bunclass a right join ctclass b on a.classid=b.classid where a.nclassid=20
查詢別名 a,b 表, 只匹配 b 表中的內容.
7.2.3 添加數據到連接表之一
程序代碼 Insert INTO cttag ( tag, articleid ) Select top 1 b.tag, a.articleid FROM ctarticle AS a left JOIN cttag AS b ON a.articleid = b.articleid Where a.articleid order by a.articleid desc;
7.2.2 變通中的用法二
程序代碼 Insert INTO bureply Select b.*, a.classid, a.nclassid FROM article AS a INNER JOIN reply AS b ON a.articleid = b.articleid Where classid=50;
7.2.1 實際應用中的變通
程序代碼 Insert INTO butag ( tag, articleid, classid, nclassid) Select b.tag, a.articleid, a.classid, a.nclassid FROM article AS a INNER JOIN tag AS b ON a.articleid = b.articleid Where classid=24;
7.2 添加數據到其他表
程序代碼 Insert INTO butag ( tag, articleid ) Select b.tag, a.articleid FROM article AS a INNER JOIN tag AS b ON a.articleid = b.articleid Where a.articleid>False;
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