MySql中如何使用JOIN
- WBOY轉載
- 2023-06-04 08:02:021301瀏覽
JOIN的意思就如英文單字「join」一樣，連接兩張表，大致分為內連接，外連接，右連接，左連接，自然連接。
先建立兩個表，以下用於範例
CREATE TABLE t_blog(
id INT PRIMARY KEY AUTO_INCREMENT,
title VARCHAR(50),
typeId INT
);
SELECT * FROM t_blog;
+----+-------+--------+
| id | title | typeId |
+----+-------+--------+
| 1 | aaa | 1 |
| 2 | bbb | 2 |
| 3 | ccc | 3 |
| 4 | ddd | 4 |
| 5 | eee | 4 |
| 6 | fff | 3 |
| 7 | ggg | 2 |
| 8 | hhh | NULL |
| 9 | iii | NULL |
| 10 | jjj | NULL |
+----+-------+--------+
-- 博客的类别
CREATE TABLE t_type(
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(20)
);
SELECT * FROM t_type;
+----+------------+
| id | name |
+----+------------+
| 1 | C++ |
| 2 | C |
| 3 | Java |
| 4 | C# |
| 5 | Javascript |
+----+------------+笛卡爾積：CROSS JOIN
要理解各種JOIN首先要理解笛卡兒積。笛卡爾積是將A表中的每個記錄與B表中的每個記錄組合。因此，當A表中有n筆記錄且B表中有m筆記錄時，笛卡爾積操作所得到的結果將會有n*m筆記錄。下面的例子，t_blog有10筆記錄，t_type有5筆記錄，所有他們兩個的笛卡爾積有50筆記錄。有五種產生笛卡兒積的方式如下。
SELECT * FROM t_blog CROSS JOIN t_type;
SELECT * FROM t_blog INNER JOIN t_type;
SELECT * FROM t_blog,t_type;
SELECT * FROM t_blog NATURE JOIN t_type;
select * from t_blog NATURA join t_type;
+----+-------+--------+----+------------+
| id | title | typeId | id | name |
+----+-------+--------+----+------------+
| 1 | aaa | 1 | 1 | C++ |
| 1 | aaa | 1 | 2 | C |
| 1 | aaa | 1 | 3 | Java |
| 1 | aaa | 1 | 4 | C# |
| 1 | aaa | 1 | 5 | Javascript |
| 2 | bbb | 2 | 1 | C++ |
| 2 | bbb | 2 | 2 | C |
| 2 | bbb | 2 | 3 | Java |
| 2 | bbb | 2 | 4 | C# |
| 2 | bbb | 2 | 5 | Javascript |
| 3 | ccc | 3 | 1 | C++ |
| 3 | ccc | 3 | 2 | C |
| 3 | ccc | 3 | 3 | Java |
| 3 | ccc | 3 | 4 | C# |
| 3 | ccc | 3 | 5 | Javascript |
| 4 | ddd | 4 | 1 | C++ |
| 4 | ddd | 4 | 2 | C |
| 4 | ddd | 4 | 3 | Java |
| 4 | ddd | 4 | 4 | C# |
| 4 | ddd | 4 | 5 | Javascript |
| 5 | eee | 4 | 1 | C++ |
| 5 | eee | 4 | 2 | C |
| 5 | eee | 4 | 3 | Java |
| 5 | eee | 4 | 4 | C# |
| 5 | eee | 4 | 5 | Javascript |
| 6 | fff | 3 | 1 | C++ |
| 6 | fff | 3 | 2 | C |
| 6 | fff | 3 | 3 | Java |
| 6 | fff | 3 | 4 | C# |
| 6 | fff | 3 | 5 | Javascript |
| 7 | ggg | 2 | 1 | C++ |
| 7 | ggg | 2 | 2 | C |
| 7 | ggg | 2 | 3 | Java |
| 7 | ggg | 2 | 4 | C# |
| 7 | ggg | 2 | 5 | Javascript |
| 8 | hhh | NULL | 1 | C++ |
| 8 | hhh | NULL | 2 | C |
| 8 | hhh | NULL | 3 | Java |
| 8 | hhh | NULL | 4 | C# |
| 8 | hhh | NULL | 5 | Javascript |
| 9 | iii | NULL | 1 | C++ |
| 9 | iii | NULL | 2 | C |
| 9 | iii | NULL | 3 | Java |
| 9 | iii | NULL | 4 | C# |
| 9 | iii | NULL | 5 | Javascript |
| 10 | jjj | NULL | 1 | C++ |
| 10 | jjj | NULL | 2 | C |
| 10 | jjj | NULL | 3 | Java |
| 10 | jjj | NULL | 4 | C# |
| 10 | jjj | NULL | 5 | Javascript |
+----+-------+--------+----+------------+內連接：INNER JOIN
內連接INNER JOIN是最常用的連接操作。從數學的角度來說，這就是要計算兩個表的相交部分；而從笛卡兒積的角度來看，就是要從笛卡兒積中篩選出滿足ON子句條件的記錄。有INNER JOIN，WHERE（等值連接），STRAIGHT_JOIN,JOIN(省略INNER)四種寫法。
SELECT * FROM t_blog INNER JOIN t_type ON t_blog.typeId=t_type.id;
SELECT * FROM t_blog,t_type WHERE t_blog.typeId=t_type.id;
SELECT * FROM t_blog STRAIGHT_JOIN t_type ON t_blog.typeId=t_type.id; --注意STRIGHT_JOIN有个下划线
SELECT * FROM t_blog JOIN t_type ON t_blog.typeId=t_type.id;
+----+-------+--------+----+------+
| id | title | typeId | id | name |
+----+-------+--------+----+------+
| 1 | aaa | 1 | 1 | C++ |
| 2 | bbb | 2 | 2 | C |
| 7 | ggg | 2 | 2 | C |
| 3 | ccc | 3 | 3 | Java |
| 6 | fff | 3 | 3 | Java |
| 4 | ddd | 4 | 4 | C# |
| 5 | eee | 4 | 4 | C# |
+----+-------+--------+----+------+左連接：LEFT JOIN
左連接LEFT JOIN的意義就是求兩個表的交集外加左表剩下的資料。依舊從笛卡兒積的角度講，就是先從笛卡兒積中挑出ON子句條件成立的記錄，然後加上左表中剩餘的記錄（見最後三條）。
SELECT * FROM t_blog LEFT JOIN t_type ON t_blog.typeId=t_type.id;
+----+-------+--------+------+------+
| id | title | typeId | id | name |
+----+-------+--------+------+------+
| 1 | aaa | 1 | 1 | C++ |
| 2 | bbb | 2 | 2 | C |
| 7 | ggg | 2 | 2 | C |
| 3 | ccc | 3 | 3 | Java |
| 6 | fff | 3 | 3 | Java |
| 4 | ddd | 4 | 4 | C# |
| 5 | eee | 4 | 4 | C# |
| 8 | hhh | NULL | NULL | NULL |
| 9 | iii | NULL | NULL | NULL |
| 10 | jjj | NULL | NULL | NULL |
+----+-------+--------+------+------+右連結：RIGHT JOIN
同理右連結RIGHT JOIN就是求兩個表的交集外加右邊表剩下的資料。再次從笛卡兒積的角度描述，右連接就是從笛卡兒積中挑出ON子句條件成立的記錄，然後加上右表中剩餘的記錄（見最後一條）。
SELECT * FROM t_blog RIGHT JOIN t_type ON t_blog.typeId=t_type.id;
+------+-------+--------+----+------------+
| id | title | typeId | id | name |
+------+-------+--------+----+------------+
| 1 | aaa | 1 | 1 | C++ |
| 2 | bbb | 2 | 2 | C |
| 3 | ccc | 3 | 3 | Java |
| 4 | ddd | 4 | 4 | C# |
| 5 | eee | 4 | 4 | C# |
| 6 | fff | 3 | 3 | Java |
| 7 | ggg | 2 | 2 | C |
| NULL | NULL | NULL | 5 | Javascript |
+------+-------+--------+----+------------+外連接：OUTER JOIN
外連接就是求兩個集合的並集。從笛卡兒積的角度講就是從笛卡兒積中挑出ON子句條件成立的記錄，然後加上左表剩餘的記錄，最後加上右表中剩餘的記錄。 MySQL不支援OUTER JOIN，但是我們可以透過將左連接和右連接的結果進行UNION操作來實現。
SELECT * FROM t_blog LEFT JOIN t_type ON t_blog.typeId=t_type.id
UNION
SELECT * FROM t_blog RIGHT JOIN t_type ON t_blog.typeId=t_type.id;
+------+-------+--------+------+------------+
| id | title | typeId | id | name |
+------+-------+--------+------+------------+
| 1 | aaa | 1 | 1 | C++ |
| 2 | bbb | 2 | 2 | C |
| 7 | ggg | 2 | 2 | C |
| 3 | ccc | 3 | 3 | Java |
| 6 | fff | 3 | 3 | Java |
| 4 | ddd | 4 | 4 | C# |
| 5 | eee | 4 | 4 | C# |
| 8 | hhh | NULL | NULL | NULL |
| 9 | iii | NULL | NULL | NULL |
| 10 | jjj | NULL | NULL | NULL |
| NULL | NULL | NULL | 5 | Javascript |
+------+-------+--------+------+------------+USING子句
MySQL中連接SQL語句中，ON子句的語法格式為：table1.column_name = table2.column_name。當模式設計對聯接表的欄位採用了相同的命名樣式時，就可以使用 USING 語法來簡化 ON 語法，格式為：USING(column_name)。
所以，USING的功能相當於ON，差別在於USING指定一個屬性名稱用來連接兩個表，而ON指定一個條件。另外，SELECT *時，USING會移除USING指定的列，而ON不會。實例如下。
SELECT * FROM t_blog INNER JOIN t_type ON t_blog.typeId =t_type.id;
+----+-------+--------+----+------+
| id | title | typeId | id | name |
+----+-------+--------+----+------+
| 1 | aaa | 1 | 1 | C++ |
| 2 | bbb | 2 | 2 | C |
| 7 | ggg | 2 | 2 | C |
| 3 | ccc | 3 | 3 | Java |
| 6 | fff | 3 | 3 | Java |
| 4 | ddd | 4 | 4 | C# |
| 5 | eee | 4 | 4 | C# |
+----+-------+--------+----+------+
SELECT * FROM t_blog INNER JOIN t_type USING(typeId);
ERROR 1054 (42S22): Unknown column 'typeId' in 'from clause'
SELECT * FROM t_blog INNER JOIN t_type USING(id); -- 应为t_blog的typeId与t_type的id不同名，无法用Using，这里用id代替下。
+----+-------+--------+------------+
| id | title | typeId | name |
+----+-------+--------+------------+
| 1 | aaa | 1 | C++ |
| 2 | bbb | 2 | C |
| 3 | ccc | 3 | Java |
| 4 | ddd | 4 | C# |
| 5 | eee | 4 | Javascript |
+----+-------+--------+------------+自然連接：NATURE JOIN
自然連接就是USING子句的簡化版，它找出兩個表中相同的列作為連接條件進行連接。有左自然連接，右自然連接和普通自然連接之分。在t_blog和t_type範例中，兩個表格相同的欄位是id，所以會拿id當連接條件。
另外千萬分清下面三條語句的差別 。
自然連結:SELECT * FROM t_blog NATURAL JOIN t_type;
笛卡兒積:SELECT * FROM t_blog NATURA JOIN t_type;
笛卡兒積:SELECT * FROM t_blog NATURE JOIN t_type;##rr
SELECT * FROM t_blog NATURAL JOIN t_type;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog,t_type WHERE t_blog.id=t_type.id;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog INNER JOIN t_type ON t_blog.id=t_type.id;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog INNER JOIN t_type USING(id);
+----+-------+--------+------------+
| id | title | typeId | name |
| 1 | aaa | 1 | C++ |
| 2 | bbb | 2 | C |
| 3 | ccc | 3 | Java |
| 4 | ddd | 4 | C# |
| 5 | eee | 4 | Javascript |
SELECT * FROM t_blog NATURAL LEFT JOIN t_type;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog LEFT JOIN t_type ON t_blog.id=t_type.id;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog LEFT JOIN t_type USING(id);
| 6 | fff | 3 | NULL |
| 7 | ggg | 2 | NULL |
| 8 | hhh | NULL | NULL |
| 9 | iii | NULL | NULL |
| 10 | jjj | NULL | NULL |
SELECT * FROM t_blog NATURAL RIGHT JOIN t_type;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog RIGHT JOIN t_type ON t_blog.id=t_type.id;
SELECT t_blog.id,title,typeId,t_type.name FROM t_blog RIGHT JOIN t_type USING(id);
+----+------------+-------+--------+
| id | name | title | typeId |
| 1 | C++ | aaa | 1 |
| 2 | C | bbb | 2 |
| 3 | Java | ccc | 3 |
| 4 | C# | ddd | 4 |
| 5 | Javascript | eee | 4 |以上是MySql中如何使用JOIN的詳細內容。更多資訊請關注PHP中文網其他相關文章！
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