MySQL：连接类型的快速细分


MySQL: Quick breakdown of the types of joins

我想快速分解MySQL连接的类型。我知道这些，其余的我不知道它们是什么意思。

  • 逗号分隔(这到底是什么意思？)：SELECT * FROM a, b WHERE b.id = a.beeId AND ...。
  • 显示来自a的信息，即使b:SELECT * FROM a LEFT OUTER JOIN b ON b.id = a.beeId WHERE ...中没有匹配项

我见过其他连接，但想知道是什么使它们不同，添加INNER或OUTER是什么改变了事情。

我已经知道连接是如何工作的，我只想知道是否还有其他类型的连接，或者它们只是获得相同结果的不同方法。

Found on G+(c)"在线可视化，发现G +日期"P＞＜／

see the following links or for a good的概述：P＞＜／

http：/ / / / /的index.php档案博客www.khankennels.com／2007／04／20／让我joins /P＞＜／

http：/ / / / / www.codinghorror.com博客2007年10 a-visual-explanation-of-sql-joins.htmlP＞＜／

相关讨论

基于你的评论，简单的定义是最好的发现在W3Schools of eacheach of the first在线简明explanation of the给A型和型P＞＜／

  • JOIN: Return rows when there is at least one match in both tables
  • LEFT JOIN: Return all rows from the left table, even if there are no matches in the right table
  • RIGHT JOIN: Return all rows from the right table, even if there are no matches in the left table
  • FULL JOIN: Return rows when there is a match in one of the tables

前端编辑P＞＜／

在nutshell，the example of拆款给你P＞＜／

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SELECT * FROM a, b WHERE b.id = a.beeId AND ...

选择的每一个记录表是从B和切割commas with the can be used the tables，这样也在列P＞＜／

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SELECT a.beeName,b.* FROM a, b WHERE b.id = a.beeId AND ...

恩，让我instructed is the information in the row where the column have a column和a.beeid b.id比赛在你的实例。我知道你会得到它的example from a和b表信息a.beeid where the b.id equals。我会得到它的example of the information from the table和B a.beename column from the only when the information the a.beeid b.id equals。注that there is also an和条款，这将帮助你的refine结果。P＞＜／

有些无知的tutorials和explanations在线joins左joins MySQL和MySQL tizag' s have a look at tutorials。You can also基思J S退房。brown'网站信息在线joins that is for黑莓也那么好。P＞＜／

希望你helps thisP＞＜／

相关讨论

全外连接MySQL中不存在，你可以用need to left和right join 1例感染。P＞＜／

2：我有这样的P＞＜／

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> SELECT * FROM table_a;
+------+------+
| id   | name |
+------+------+
|    1 | row1 |
|    2 | row2 |
+------+------+

> SELECT * FROM table_b;
+------+------+------+
| id   | name | aid  |
+------+------+------+
|    3 | row3 |    1 |
|    4 | row4 |    1 |
|    5 | row5 | NULL |
+------+------+------+

内部连接的两表关心。P＞＜／

关于两表内加入的关心，如果你只得到一个两行的表。if there is超过一对多匹配，你就行。P＞＜／

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> SELECT * FROM table_a a INNER JOIN table_b b ON a.id=b.aid;
+------+------+------+------+------+
| id   | name | id   | name | aid  |
+------+------+------+------+------+
|    1 | row1 |    3 | row3 | 1    |
|    1 | row1 |    4 | row4 | 1    |
+------+------+------+------+------+

它使不差to join the如果你逆向内秩序，因为它关心的：both aboutP＞＜／

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> SELECT * FROM table_b b INNER JOIN table_a a ON a.id=b.aid;
+------+------+------+------+------+
| id   | name | aid  | id   | name |
+------+------+------+------+------+
|    3 | row3 | 1    |    1 | row1 |
|    4 | row4 | 1    |    1 | row1 |
+------+------+------+------+------+

You get the same but the columns are rows，因为我们在不同的阶阶在上述不同的茶。P＞＜／

left join table about the第一只关心P＞＜／

about the first left join table的关心你给它，和不在乎多about the second，你总是知道get the rows if from the table是第一，there is the second：在对应的行不P＞＜／

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> SELECT * FROM table_a a LEFT JOIN table_b b ON a.id=b.aid;
+------+------+------+------+------+
| id   | name | id   | name | aid  |
+------+------+------+------+------+
|    1 | row1 |    3 | row3 | 1    |
|    1 | row1 |    4 | row4 | 1    |
|    2 | row2 | NULL | NULL | NULL |
+------+------+------+------+------+

你可以看到在上面的表_ rows of some of them a即使在table with do not match anything but not B，在B - _ rows of ones that table table的东西只_在比赛中。P＞＜／

if we the Order of the tables逆向，left join behaves differently：P＞＜／

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> SELECT * FROM table_b b LEFT JOIN table_a a ON a.id=b.aid;
+------+------+------+------+------+
| id   | name | aid  | id   | name |
+------+------+------+------+------+
|    3 | row3 | 1    |    1 | row1 |
|    4 | row4 | 1    |    1 | row1 |
|    5 | row5 | NULL | NULL | NULL |
+------+------+------+------+------+

现在我们在_ rows of get table匹配B，but only _ rows of table。P＞＜／

right join table about the second only的关心P＞＜／

你a RIGHT JOIN b得到确切的说因为是the same as b LEFT JOIN arows。差分阶the only is the columns of the默认。P＞＜／

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> SELECT * FROM table_a a RIGHT JOIN table_b b ON a.id=b.aid;
+------+------+------+------+------+
| id   | name | id   | name | aid  |
+------+------+------+------+------+
|    1 | row1 |    3 | row3 | 1    |
|    1 | row1 |    4 | row4 | 1    |
| NULL | NULL |    5 | row5 | NULL |
+------+------+------+------+------+

This is the same as table_b LEFT JOIN table_arows，which we the left join在截面看到。P＞＜／

similarly：P＞＜／

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> SELECT * FROM table_b b RIGHT JOIN table_a a ON a.id=b.aid;
+------+------+------+------+------+
| id   | name | aid  | id   | name |
+------+------+------+------+------+
|    3 | row3 | 1    |    1 | row1 |
|    4 | row4 | 1    |    1 | row1 |
| NULL | NULL | NULL |    2 | row2 |
+------+------+------+------+------+

is the same as table_a LEFT JOIN table_brows。P＞＜／

不给你拷贝用at all of MindP＞＜／

如果你不写你的表与连接在所有的条款，只是commas separated by，You get table of the first所写的每行每行of the second下表，在每个可能的组合：P＞＜／

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> SELECT * FROM table_b b, table_a;
+------+------+------+------+------+
| id   | name | aid  | id   | name |
+------+------+------+------+------+
|    3 | row3 | 1    |    1 | row1 |
|    3 | row3 | 1    |    2 | row2 |
|    4 | row4 | 1    |    1 | row1 |
|    4 | row4 | 1    |    2 | row2 |
|    5 | row5 | NULL |    1 | row1 |
|    5 | row5 | NULL |    2 | row2 |
+------+------+------+------+------+

(this is from examples of SQL连接我的博客类型)P＞＜／