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MySQLStudy之--MySQL表連接_MySQL

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MySQLStudy之--MySQL表連接_MySQL

MySQLStudy之--MySQL表連接_MySQL:MySQL Study之--MySQL 表連接一．Join語法概述 join 用于多表中字段之間的聯系，語法如下： ... FROM table1 INNER|LEFT|RIGHT JOIN table2 ON condition table1:左表；table2:右表。 JOIN 按照功能大致分為如下三類： I

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導讀MySQLStudy之--MySQL表連接_MySQL:MySQL Study之--MySQL 表連接一．Join語法概述 join 用于多表中字段之間的聯系，語法如下： ... FROM table1 INNER|LEFT|RIGHT JOIN table2 ON condition table1:左表；table2:右表。 JOIN 按照功能大致分為如下三類： I

MySQL Study之--MySQL 表連接

一．Join語法概述

join 用于多表中字段之間的聯系，語法如下：

... FROM table1 INNER|LEFT|RIGHT JOIN table2 ON condition

table1:左表；table2:右表。

JOIN 按照功能大致分為如下三類：

INNER JOIN（內連接,或等值連接）：取得兩個表中存在連接匹配關系的記錄。

LEFT JOIN（左連接）：取得左表（table1）完全記錄，即是右表（table2）并無對應匹配記錄。

RIGHT JOIN（右連接）：與 LEFT JOIN 相反，取得右表（table2）完全記錄，即是左表（table1）并無匹配對應記錄。

注意：mysql不支持Full join,不過可以通過UNION 關鍵字來合并 LEFT JOIN 與 RIGHT JOIN來模擬FULL join.

案例分析：

1、案例環境

mysql> select * from emp;
+-------+--------+-----------+------+------------+------+------+--------+
| empno | ENAME | JOB | MGR | HIRE | SAL | COMM | deptno |
+-------+--------+-----------+------+------------+------+------+--------+
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800 | NULL | 20 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600 | 300 | 30 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250 | 500 | 30 |
| 7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975 | NULL | 20 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250 | 1400 | 30 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 | 2850 | NULL | 30 |
| 7782 | CLARK | MANAGER | 7839 | 1981-06-09 | 2450 | NULL | 10 |
| 7788 | SCOTT | ANALYST | 7566 | 1987-07-13 | 3000 | NULL | 20 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000 | NULL | 10 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500 | 0 | 30 |
| 7876 | ADAMS | CLERK | 7788 | 1987-06-13 | 1100 | NULL | 20 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950 | NULL | 30 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000 | NULL | 20 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300 | NULL | 10 |
+-------+--------+-----------+------+------------+------+------+--------+
14 rows in set (0.00 sec)

mysql> select * from dept;
+--------+------------+---------+
| deptNO | DNAME | LOC |
+--------+------------+---------+
| 10 | ACCOUNTING | NEWYORK |
| 20 | RESEARCH | DALLAS |
| 30 | SALES | CHICAGO |
| 40 | OPERATIONS | BOSTON |
+--------+------------+---------+
4 rows in set (0.00 sec)

inner join：（內連接,或等值連接）：取得兩個表中存在連接匹配關系的記錄。

mysql> select e.empno,e.ename,e.sal,e.deptno,d.dname 
 -> from emp e
 -> inner join dept d 
 -> where e.deptno=d.deptno;
+-------+--------+------+--------+------------+
| empno | ename | sal | deptno | dname |
+-------+--------+------+--------+------------+
| 7782 | CLARK | 2450 | 10 | ACCOUNTING |
| 7839 | KING | 5000 | 10 | ACCOUNTING |
| 7934 | MILLER | 1300 | 10 | ACCOUNTING |
| 7369 | SMITH | 800 | 20 | RESEARCH |
| 7566 | JONES | 2975 | 20 | RESEARCH |
| 7788 | SCOTT | 3000 | 20 | RESEARCH |
| 7876 | ADAMS | 1100 | 20 | RESEARCH |
| 7902 | FORD | 3000 | 20 | RESEARCH |
| 7499 | ALLEN | 1600 | 30 | SALES |
| 7521 | WARD | 1250 | 30 | SALES |
| 7654 | MARTIN | 1250 | 30 | SALES |
| 7698 | BLAKE | 2850 | 30 | SALES |
| 7844 | TURNER | 1500 | 30 | SALES |
| 7900 | JAMES | 950 | 30 | SALES |
+-------+--------+------+--------+------------+
14 rows in set (0.00 sec)

mysql> select e.empno,e.ename,e.sal,e.deptno,d.dname 
 -> from emp e
 -> inner join dept d 
 -> on e.deptno=d.deptno;
+-------+--------+------+--------+------------+
| empno | ename | sal | deptno | dname |
+-------+--------+------+--------+------------+
| 7782 | CLARK | 2450 | 10 | ACCOUNTING |
| 7839 | KING | 5000 | 10 | ACCOUNTING |
| 7934 | MILLER | 1300 | 10 | ACCOUNTING |
| 7369 | SMITH | 800 | 20 | RESEARCH |
| 7566 | JONES | 2975 | 20 | RESEARCH |
| 7788 | SCOTT | 3000 | 20 | RESEARCH |
| 7876 | ADAMS | 1100 | 20 | RESEARCH |
| 7902 | FORD | 3000 | 20 | RESEARCH |
| 7499 | ALLEN | 1600 | 30 | SALES |
| 7521 | WARD | 1250 | 30 | SALES |
| 7654 | MARTIN | 1250 | 30 | SALES |
| 7698 | BLAKE | 2850 | 30 | SALES |
| 7844 | TURNER | 1500 | 30 | SALES |
| 7900 | JAMES | 950 | 30 | SALES |
+-------+--------+------+--------+------------+
14 rows in set (0.00 sec)

隱式內連接：

mysql> select e.empno,e.ename,e.sal,e.deptno,d.dname 
 -> from emp e,dept d
 -> where e.deptno=d.deptno;
+-------+--------+------+--------+------------+
| empno | ename | sal | deptno | dname |
+-------+--------+------+--------+------------+
| 7782 | CLARK | 2450 | 10 | ACCOUNTING |
| 7839 | KING | 5000 | 10 | ACCOUNTING |
| 7934 | MILLER | 1300 | 10 | ACCOUNTING |
| 7369 | SMITH | 800 | 20 | RESEARCH |
| 7566 | JONES | 2975 | 20 | RESEARCH |
| 7788 | SCOTT | 3000 | 20 | RESEARCH |
| 7876 | ADAMS | 1100 | 20 | RESEARCH |
| 7902 | FORD | 3000 | 20 | RESEARCH |
| 7499 | ALLEN | 1600 | 30 | SALES |
| 7521 | WARD | 1250 | 30 | SALES |
| 7654 | MARTIN | 1250 | 30 | SALES |
| 7698 | BLAKE | 2850 | 30 | SALES |
| 7844 | TURNER | 1500 | 30 | SALES |
| 7900 | JAMES | 950 | 30 | SALES |
+-------+--------+------+--------+------------+
14 rows in set (0.00 sec)

left join：取得左表（table1）完全記錄，即是右表（table2）并無對應匹配記錄。

mysql> update emp set deptno=null where empno=7788;
Query OK, 1 row affected (0.07 sec)
Rows matched: 1 Changed: 1 Warnings: 0

mysql> commit;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from emp where empno=7788;
+-------+-------+---------+------+------------+------+------+--------+
| empno | ENAME | JOB | MGR | HIRE | SAL | COMM | deptno |
+-------+-------+---------+------+------------+------+------+--------+
| 7788 | SCOTT | ANALYST | 7566 | 1987-07-13 | 3000 | NULL | NULL |
+-------+-------+---------+------+------------+------+------+--------+
1 row in set (0.00 sec)

采用等值連接：

mysql> select e.empno,e.ename,e.sal,e.deptno,d.dname
 -> from emp e
 -> inner join dept d on e.deptno=d.deptno;
+-------+--------+------+--------+------------+
| empno | ename | sal | deptno | dname |
+-------+--------+------+--------+------------+
| 7782 | CLARK | 2450 | 10 | ACCOUNTING |
| 7839 | KING | 5000 | 10 | ACCOUNTING |
| 7934 | MILLER | 1300 | 10 | ACCOUNTING |
| 7369 | SMITH | 800 | 20 | RESEARCH |
| 7566 | JONES | 2975 | 20 | RESEARCH |
| 7876 | ADAMS | 1100 | 20 | RESEARCH |
| 7902 | FORD | 3000 | 20 | RESEARCH |
| 7499 | ALLEN | 1600 | 30 | SALES |
| 7521 | WARD | 1250 | 30 | SALES |
| 7654 | MARTIN | 1250 | 30 | SALES |
| 7698 | BLAKE | 2850 | 30 | SALES |
| 7844 | TURNER | 1500 | 30 | SALES |
| 7900 | JAMES | 950 | 30 | SALES |
+-------+--------+------+--------+------------+
13 rows in set (0.00 sec)

-----對于等值連接，只能看到條件匹配的記錄！

mysql> select e.empno,e.ename,e.sal,e.deptno,d.dname
 -> from emp e
 -> left join dept d on e.deptno=d.deptno;
+-------+--------+------+--------+------------+
| empno | ename | sal | deptno | dname |
+-------+--------+------+--------+------------+
| 7782 | CLARK | 2450 | 10 | ACCOUNTING |
| 7839 | KING | 5000 | 10 | ACCOUNTING |
| 7934 | MILLER | 1300 | 10 | ACCOUNTING |
| 7369 | SMITH | 800 | 20 | RESEARCH |
| 7566 | JONES | 2975 | 20 | RESEARCH |
| 7876 | ADAMS | 1100 | 20 | RESEARCH |
| 7902 | FORD | 3000 | 20 | RESEARCH |
| 7499 | ALLEN | 1600 | 30 | SALES |
| 7521 | WARD | 1250 | 30 | SALES |
| 7654 | MARTIN | 1250 | 30 | SALES |
| 7698 | BLAKE | 2850 | 30 | SALES |
| 7844 | TURNER | 1500 | 30 | SALES |
| 7900 | JAMES | 950 | 30 | SALES |
| 7788 | SCOTT | 3000 | NULL | NULL |
+-------+--------+------+--------+------------+
14 rows in set (0.00 sec)

-----通過left join可以查看到emp表中不符合條件的記錄！

right join：與 LEFT JOIN 相反，取得右表（table2）完全記錄，即是左表（table1）并無匹配對應記錄。

mysql> select e.empno,e.ename,e.sal,d.deptno,d.dname
 -> from emp e
 -> right join dept d on e.deptno=d.deptno;
+-------+--------+------+--------+------------+
| empno | ename | sal | deptno | dname |
+-------+--------+------+--------+------------+
| 7782 | CLARK | 2450 | 10 | ACCOUNTING |
| 7839 | KING | 5000 | 10 | ACCOUNTING |
| 7934 | MILLER | 1300 | 10 | ACCOUNTING |
| 7369 | SMITH | 800 | 20 | RESEARCH |
| 7566 | JONES | 2975 | 20 | RESEARCH |
| 7876 | ADAMS | 1100 | 20 | RESEARCH |
| 7902 | FORD | 3000 | 20 | RESEARCH |
| 7499 | ALLEN | 1600 | 30 | SALES |
| 7521 | WARD | 1250 | 30 | SALES |
| 7654 | MARTIN | 1250 | 30 | SALES |
| 7698 | BLAKE | 2850 | 30 | SALES |
| 7844 | TURNER | 1500 | 30 | SALES |
| 7900 | JAMES | 950 | 30 | SALES |
| NULL | NULL | NULL | 40 | OPERATIONS |
+-------+--------+------+--------+------------+
14 rows in set (0.00 sec)

------查詢到dept表中，不符合條件的記錄！！！

淺析Mysql Join語法以及性能優化

在講MySQL的Join語法前還是先回顧一下聯結的語法，呵呵，其實連我自己都忘得差不多了，那就大家一起溫習吧，這里我有個比較簡便的記憶方法，內外聯結的區別是內聯結將去除所有不符合條件的記錄，而外聯結則保留其中部分。外左聯結與外右聯結的區別在于如果用A左聯結B則A中所有記錄都會保留在結果中，此時B中只有符合聯結條件的記錄，而右聯結相反，這樣也就不會混淆

了。

一．Join語法概述

join 用于多表中字段之間的聯系，語法如下：

... FROM table1 INNER|LEFT|RIGHT JOIN table2 ON conditiona

table1:左表；table2:右表。

JOIN 按照功能大致分為如下三類：

INNER JOIN（內連接,或等值連接）：取得兩個表中存在連接匹配關系的記錄。

LEFT JOIN（左連接）：取得左表（table1）完全記錄，即是右表（table2）并無對應匹配記錄。

RIGHT JOIN（右連接）：與 LEFT JOIN 相反，取得右表（table2）完全記錄，即是左表（table1）并無匹配對應記錄。

注意：mysql不支持Full join,不過可以通過UNION 關鍵字來合并 LEFT JOIN 與 RIGHT JOIN來模擬FULL join.

接下來給出一個列子用于解釋下面幾種分類。如下兩個表(A,B)

mysql> select A.id,A.name,B.name from A,B where A.id=B.id;
+----+-----------+-------------+
| id | name | name |
+----+-----------+-------------+
| 1 | Pirate | Rutabaga |
| 2 | Monkey | Pirate |
| 3 | Ninja | Darth Vader |
| 4 | Spaghetti | Ninja |
+----+-----------+-------------+
4 rows in set (0.00 sec)

二.Inner join

內連接，也叫等值連接，inner join產生同時符合A和B的一組數據。

mysql> select * from A inner join B on A.name = B.name;
+----+--------+----+--------+
| id | name | id | name |
+----+--------+----+--------+
| 1 | Pirate | 2 | Pirate |
| 3 | Ninja | 4 | Ninja |
+----+--------+----+--------+

三.Left join

mysql> select * from A left join B on A.name = B.name;
#或者：select * from A left outer join B on A.name = B.name;
+----+-----------+------+--------+
| id | name | id | name |
+----+-----------+------+--------+
| 1 | Pirate | 2 | Pirate |
| 2 | Monkey | NULL | NULL |
| 3 | Ninja | 4 | Ninja |
| 4 | Spaghetti | NULL | NULL |
+----+-----------+------+--------+
4 rows in set (0.00 sec)

left join,（或left outer join:在Mysql中兩者等價，推薦使用left join.）左連接從左表(A)產生一套完整的記錄,與匹配的記錄(右表(B)) .如果沒有匹配,右側將包含null

如果想只從左表(A)中產生一套記錄，但不包含右表(B)的記錄，可以通過設置where語句來執行，如下：

mysql> select * from A left join B on A.name=B.name where A.id is null or B.id is null;
+----+-----------+------+------+
| id | name | id | name |
+----+-----------+------+------+
| 2 | Monkey | NULL | NULL |
| 4 | Spaghetti | NULL | NULL |
+----+-----------+------+------+
2 rows in set (0.00 sec)

同理，還可以模擬inner join. 如下：

mysql> select * from A left join B on A.name=B.name where A.id is not null and B.id is not null;
+----+--------+------+--------+
| id | name | id | name |
+----+--------+------+--------+
| 1 | Pirate | 2 | Pirate |
| 3 | Ninja | 4 | Ninja |
+----+--------+------+--------+
2 rows in set (0.00 sec)

求差集：

根據上面的例子可以求差集，如下：

SELECT * FROM A LEFT JOIN B ON A.name = B.name
WHERE B.id IS NULL
union
SELECT * FROM A right JOIN B ON A.name = B.name
WHERE A.id IS NULL;

# 結果
 +------+-----------+------+-------------+
| id | name | id | name |
+------+-----------+------+-------------+
| 2 | Monkey | NULL | NULL |
| 4 | Spaghetti | NULL | NULL |
| NULL | NULL | 1 | Rutabaga |
| NULL | NULL | 3 | Darth Vader |
+------+-----------+------+-------------+

四.Right join

mysql> select * from A right join B on A.name = B.name;
+------+--------+----+-------------+
| id | name | id | name |
+------+--------+----+-------------+
| NULL | NULL | 1 | Rutabaga |
| 1 | Pirate | 2 | Pirate |
| NULL | NULL | 3 | Darth Vader |
| 3 | Ninja | 4 | Ninja |
+------+--------+----+-------------+
4 rows in set (0.00 sec)

同left join。

五.Cross join

cross join：交叉連接，得到的結果是兩個表的乘積，即笛卡爾積

笛卡爾（Descartes）乘積又叫直積。假設集合A={a,b}，集合B={0,1,2}，則兩個集合的笛卡爾積為{(a,0),(a,1),(a,2),(b,0),(b,1), (b,2)}。可以擴展到多個集合的情況。類似的例子有，如果A表示某學校學生的集合，B表示該學校所有課程的集合，則A與B的笛卡爾積表示所有可能的選課情況。

mysql> select * from A cross join B;
+----+-----------+----+-------------+
| id | name | id | name |
+----+-----------+----+-------------+
| 1 | Pirate | 1 | Rutabaga |
| 2 | Monkey | 1 | Rutabaga |
| 3 | Ninja | 1 | Rutabaga |
| 4 | Spaghetti | 1 | Rutabaga |
| 1 | Pirate | 2 | Pirate |
| 2 | Monkey | 2 | Pirate |
| 3 | Ninja | 2 | Pirate |
| 4 | Spaghetti | 2 | Pirate |
| 1 | Pirate | 3 | Darth Vader |
| 2 | Monkey | 3 | Darth Vader |
| 3 | Ninja | 3 | Darth Vader |
| 4 | Spaghetti | 3 | Darth Vader |
| 1 | Pirate | 4 | Ninja |
| 2 | Monkey | 4 | Ninja |
| 3 | Ninja | 4 | Ninja |
| 4 | Spaghetti | 4 | Ninja |
+----+-----------+----+-------------+
16 rows in set (0.00 sec)

#再執行：mysql> select * from A inner join B; 試一試

#在執行mysql> select * from A cross join B on A.name = B.name; 試一試

實際上，在 MySQL 中（僅限于 MySQL） CROSS JOIN 與 INNER JOIN 的表現是一樣的，在不指定 ON 條件得到的結果都是笛卡爾積，反之取得兩個表完全匹配的結果。 INNER JOIN 與 CROSS JOIN 可以省略 INNER 或 CROSS 關鍵字，因此下面的 SQL 效果是一樣的：

... FROM table1 INNER JOIN table2

... FROM table1 CROSS JOIN table2

... FROM table1 JOIN table2

六.Full join

mysql> select * from A left join B on B.name = A.name 
 -> union 
 -> select * from A right join B on B.name = A.name;
+------+-----------+------+-------------+
| id | name | id | name |
+------+-----------+------+-------------+
| 1 | Pirate | 2 | Pirate |
| 2 | Monkey | NULL | NULL |
| 3 | Ninja | 4 | Ninja |
| 4 | Spaghetti | NULL | NULL |
| NULL | NULL | 1 | Rutabaga |
| NULL | NULL | 3 | Darth Vader |
+------+-----------+------+-------------+
6 rows in set (0.00 sec)

全連接產生的所有記錄（雙方匹配記錄）在表A和表B。如果沒有匹配,則對面將包含null。

七.性能優化

1.顯示(explicit) inner join VS 隱式(implicit) inner join

如：

select * from

table a inner join table b

on a.id = b.id;

select a.*, b.*

from table a, table b

where a.id = b.id;

我在數據庫中比較(10w數據)得之，它們用時幾乎相同，第一個是顯示的inner join，后一個是隱式的inner join。

2.left join/right join VS inner join

盡量用inner join.避免 LEFT JOIN 和 NULL.

在使用left join（或right join）時，應該清楚的知道以下幾點：

(1). on與 where的執行順序

ON 條件（“A LEFT JOIN B ON 條件表達式”中的ON）用來決定如何從 B 表中檢索數據行。如果 B 表中沒有任何一行數據匹配 ON 的條件,將會額外生成一行所有列為 NULL 的數據,在匹配階段 WHERE 子句的條件都不會被使用。僅在匹配階段完成以后，WHERE 子句條件才會被使用。它將從匹配階段產生的數據中檢索過濾。

所以我們要注意：在使用Left (right) join的時候，一定要在先給出盡可能多的匹配滿足條件，減少Where的執行。如：

PASS
select * from A
inner join B on B.name = A.name
left join C on C.name = B.name
left join D on D.id = C.id
where C.status>1 and D.status=1;

select * from A
inner join B on B.name = A.name
left join C on C.name = B.name and C.status>1
left join D on D.id = C.id and D.status=1

從上面例子可以看出，盡可能滿足ON的條件，而少用Where的條件。從執行性能來看第二個顯然更加省時。

(2).注意ON 子句和 WHERE 子句的不同

如作者舉了一個列子：

mysql> SELECT * FROM product LEFT JOIN product_details
 ON (product.id = product_details.id)
 AND product_details.id=2;
+----+--------+------+--------+-------+
| id | amount | id | weight | exist |
+----+--------+------+--------+-------+
| 1 | 100 | NULL | NULL | NULL |
| 2 | 200 | 2 | 22 | 0 |
| 3 | 300 | NULL | NULL | NULL |
| 4 | 400 | NULL | NULL | NULL |
+----+--------+------+--------+-------+
4 rows in set (0.00 sec)
mysql> SELECT * FROM product LEFT JOIN product_details
 ON (product.id = product_details.id)
 WHERE product_details.id=2;
+----+--------+----+--------+-------+
| id | amount | id | weight | exist |
+----+--------+----+--------+-------+
| 2 | 200 | 2 | 22 | 0 |
+----+--------+----+--------+-------+
1 row in set (0.01 sec)

從上可知，第一條查詢使用 ON 條件決定了從 LEFT JOIN的 product_details表中檢索符合的所有數據行。第二條查詢做了簡單的LEFT JOIN，然后使用 WHERE 子句從 LEFT JOIN的數據中過濾掉不符合條件的數據行。

(3).盡量避免子查詢，而用join

往往性能這玩意兒，更多時候體現在數據量比較大的時候，此時，我們應該避免復雜的子查詢。如下：

insert into t1(a1) select b1 from t2 where not exists(select 1 from t1 where t1.id = t2.r_id);

insert into t1(a1)

select b1 from t2

left join (select distinct t1.id from t1 ) t1 on t1.id = t2.r_id

where t1.id is null;

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