SQL连接方式（左连接、右连接、全连接）

1.内连接、左连接、右连接、全连接介绍

<wbr><wbr><wbr>內连接仅选出两张表中互相匹配的记录．因此，这会导致有时我们需要的记录没有包含进来。内部连接是两个表中都必须有连接字段的对应值的记录，数据才能检索出来。<br><wbr><wbr><wbr>左连接和右连接都是外部连接，也就是区别于内部连接，它对不满足连接条件的行并不是象内部连接一样将数据完全过滤掉，而是保留一部分数据，行数不会减少。<br><wbr>左连接是只要左边表中有记录，数据就能检索出来，而右边有的记录必要在左边表中有的记录才能被检索出来<br><wbr>右连接是只要右边表中有记录，数据就能检索出来 ；<br><wbr>全连接则会回返回两个表中的所有记录。<br><wbr><br> 2.示例<br><wbr>有两个表<br><wbr>table1<br><wbr>user_id<wbr><wbr><wbr><wbr><wbr><wbr>user_name<wbr><wbr><wbr><wbr><wbr><wbr>user_pss<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr>1<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>aaa<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>a<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr>2<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>bbb<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>b<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr>3<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>ccc<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>c<br><wbr><br><wbr>table2<br><wbr>user_id<wbr><wbr><wbr><wbr><wbr><wbr>user_power<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr>1<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>111000<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr>2<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>000111</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

<wbr>使用sql語句查询<br><wbr>正常状态:<br><wbr>select table1.user_id,<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>table1.user_name,<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>table2.user_power<br><wbr>from table1,table2<br><wbr>where table1.user_id = table2.user_id<br><wbr>但是这样检索到的数据只有两条:<br><wbr>user_id<wbr><wbr><wbr><wbr><wbr><wbr>user_name<wbr><wbr><wbr><wbr><wbr><wbr><wbr>user_power<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr>1<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>aaa<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>000111<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr>2<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>bbb<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>111000<br><wbr>因为第三条数据在table2中沒有关联.</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

<wbr>这时使用左连接查询﹕<br><wbr>selecttable1.user_id,table1.user_name,table2.user_power<br><wbr>from table1,table2<br><wbr>where table1.user_id (+)= table2.user_id<br><wbr>就可以得到如下的结果<br><wbr>user_id<wbr><wbr><wbr><wbr><wbr><wbr>user_name<wbr><wbr><wbr><wbr><wbr><wbr><wbr>user_power<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr>1<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>aaa<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>000111<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr>2<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>bbb<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>111000<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr>3<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>ccc<wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>(null)<br><wbr><wbr><wbr><wbr><wbr><wbr><wbr><br><wbr>说明﹐左或右连接查询实际上是指定以哪个表的数据为准﹐而默认（不指定左或右连接）是以两个表中都存在关键列的数据为准。<br><wbr>左连接就是以左边的表记录为主，右连接就是以右边的表记录为主，完全连接返回两个表中的所有记录。</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

3. sql写法
<wbr>内连接inner join：<br><wbr><wbr> SELECT msp.name,party.name<br><wbr>FROM msp JOIN party ON party=code<br><wbr> 或<br><wbr><wbr> SELECT msp.name,party.name<br><wbr><wbr> FROM msp inner JOIN party ONparty=code<br><wbr><br><wbr> 左连接left join ：<br><wbr><wbr>SELECT msp.name,party.name<br><wbr>FROM msp LEFT JOIN party ON party=code<br><wbr><br><wbr> 右连接right join ：<br><wbr><wbr> SELECT msp.name,party.name<br><wbr>FROM msp RIGHT JOIN party ONmsp.party=party.code</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

全连接(full join)：
<wbr>SELECT msp.name, party.name<br><wbr>FROM msp FULL JOIN party ONmsp.party=party.code</wbr></wbr>