MySQL Study之--MySQL 表連接

MySQL Study之--MySQL 表連接

MySQL Study之--MySQL 表連接   一．Join語法概述   join 用於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）並無對應匹配記錄。   RIGHT JOIN（右連接）：與 LEFT JOIN 相反，取得右表（table2）完全記錄，即是左表（table1）並無匹配對應記錄。   注意：mysql不支持Full join,不過可以通過UNION 關鍵字來合並 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）並無對應匹配記錄。

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)

 

-----對於等值連接，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）並無匹配對應記錄。  

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，這裡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於如果用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 用於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）並無對應匹配記錄。   RIGHT JOIN（右連接）：與 LEFT JOIN 相反，取得右表（table2）完全記錄，即是左表（table1）並無匹配對應記錄。   注意：mysql不支持Full join,不過可以通過UNION 關鍵字來合並 LEFT JOIN 與 RIGHT JOIN來模擬FULL join.   接下來給出一個列子用於解釋下面幾種分類。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（僅限於 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; VS 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，後一個是隱式的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後，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，然後使用 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;