好了终于贴完了MySQL联合查询的内容了,加上上一篇一共2篇,都是我转载的,实例讲解MySQL联合查询.那下面就具体讲讲简单的JOIN的用法
1. 内联结:
Select A.Name B.Hobby from A, B where A.id = B.id,这是隐式的内联结,查询的结果是:
Name Hobby
Tim Football
Jimmy Basketball
Jimmy Tennis
Tom Soccer
它的作用和 Select A.Name from A INNER JOIN B ON A.id = B.id是一样的。这里的INNER JOIN换成CROSS JOIN也是可以的。
2. 外左联结
Select A.Name from A Left JOIN B ON A.id = B.id,典型的外左联结,这样查询得到的结果将会是保留所有A表中联结字段的记录,若无与其相对应的B表中的字段记录则留空,结果如下:
Name Hobby
Tim Football
Jimmy Basketball,Tennis
John
Tom Soccer
所以从上面结果看出,因为A表中的John记录的ID没有在B表中有对应ID,因此为空,但Name栏仍有John记录。
3. 外右联结
如果把上面查询改成外右联结:Select A.Name from A Right JOIN B ON A.id = B.id,则结果将会是:
Name Hobby
Tim Football
Jimmy Basketball
Jimmy Tennis
Tom Soccer
这样的结果都是我们可以从外左联结的结果中猜到的了。
说到这里大家是否对联结查询了解多了?这个原本看来高深的概念一下子就理解了,恍然大悟了吧(呵呵,开玩笑了)?最后给大家讲讲MySQL联结查询中的某些参数的作用:
1.USING (column_list):其作用是为了方便书写联结的多对应关系,大部分情况下USING语句可以用ON语句来代替,如下面例子:
a LEFT JOIN b USING (c1,c2,c3),其作用相当于下面语句
a LEFT JOIN b ON a.c1=b.c1 AND a.c2=b.c2 AND a.c3=b.c3
只是用ON来代替会书写比较麻烦而已。
2.NATURAL [LEFT] JOIN:这个句子的作用相当于INNER JOIN,或者是在USING子句中包含了联结的表中所有字段的Left JOIN(左联结)。
3.STRAIGHT_JOIN:由于默认情况下MySQL在进行表的联结的时候会先读入左表,当使用了这个参数后MySQL将会先读入右表,这是个MySQL的内置优化参数,大家应该在特定情况下使用,譬如已经确认右表中的记录数量少,在筛选后能大大提高查询速度。
最后要说的就是,在MySQL5.0以后,运算顺序得到了重视,所以对多表的联结查询可能会错误以子联结查询的方式进行。譬如你需要进行多表联结,,因此你输入了下面的联结查询:
SELECT t1.id,t2.id,t3.id
FROM t1,t2
LEFT JOIN t3 ON (t3.id=t1.id)
WHERE t1.id=t2.id;
但是MySQL并不是这样执行的,其后台的真正执行方式是下面的语句:
SELECT t1.id,t2.id,t3.id
FROM t1,( t2 LEFT JOIN t3 ON (t3.id=t1.id) )
WHERE t1.id=t2.id;
这并不是我们想要的效果,所以我们需要这样输入:
SELECT t1.id,t2.id,t3.id
FROM (t1,t2)
LEFT JOIN t3 ON (t3.id=t1.id)
WHERE t1.id=t2.id;
在这里括号是相当重要的,因此以后在写这样的查询的时候我们不要忘记了多写几个括号,至少这样能避免很多错误(因为这样的错误是很难被开发人员发现的)。

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