最新文章專題視頻專題問答1問答10問答100問答1000問答2000關(guān)鍵字專題1關(guān)鍵字專題50關(guān)鍵字專題500關(guān)鍵字專題1500TAG最新視頻文章推薦1 推薦3 推薦5 推薦7 推薦9 推薦11 推薦13 推薦15 推薦17 推薦19 推薦21 推薦23 推薦25 推薦27 推薦29 推薦31 推薦33 推薦35 推薦37視頻文章20視頻文章30視頻文章40視頻文章50視頻文章60 視頻文章70視頻文章80視頻文章90視頻文章100視頻文章120視頻文章140 視頻2關(guān)鍵字專題關(guān)鍵字專題tag2tag3文章專題文章專題2文章索引1文章索引2文章索引3文章索引4文章索引5123456789101112131415文章專題3
問答文章1 問答文章501 問答文章1001 問答文章1501 問答文章2001 問答文章2501 問答文章3001 問答文章3501 問答文章4001 問答文章4501 問答文章5001 問答文章5501 問答文章6001 問答文章6501 問答文章7001 問答文章7501 問答文章8001 問答文章8501 問答文章9001 問答文章9501
當(dāng)前位置: 首頁 - 科技 - 知識百科 - 正文

MySQLStudy之--MySQL表連接_MySQL

來源:懂視網(wǎng) 責(zé)編:小采 時(shí)間:2020-11-09 20:13:37
文檔

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(lián)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
推薦度:
導(dǎo)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(lián)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(lián)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(nèi)連接,或等值連接):取得兩個(gè)表中存在連接匹配關(guān)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(yīng)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(yīng)記錄。

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

案例分析:

1、案例環(huán)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(nèi)連接,或等值連接):取得兩個(gè)表中存在連接匹配關(guān)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(nèi)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(yīng)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(yīng)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(yōu)化

在講MySQL的Join語法前還是先回顧一下聯(lián)結(jié)的語法,呵呵,其實(shí)連我自己都忘得差不多了,那就大家一起溫習(xí)吧,這里我有個(gè)比較簡便的記憶方法,內(nèi)外聯(lián)結(jié)的區(qū)別是內(nèi)聯(lián)結(jié)將去除所有不符合條件的記錄,而外聯(lián)結(jié)則保留其中部分。外左聯(lián)結(jié)與外右聯(lián)結(jié)的區(qū)別在于如果用A左聯(lián)結(jié)B則A中所有記錄都會保留在結(jié)果中,此時(shí)B中只有符合聯(lián)結(jié)條件的記錄,而右聯(lián)結(jié)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(lián)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(nèi)連接,或等值連接):取得兩個(gè)表中存在連接匹配關(guān)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(yīng)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(yīng)記錄。

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

接下來給出一個(gè)列子用于解釋下面幾種分類。如下兩個(gè)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(nèi)連接,也叫等值連接,inner join產(chǎn)生同時(shí)符合A和B的一組數(shù)據(jù)。

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(jià),推薦使用left join.)左連接從左表(A)產(chǎn)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(cè)將包含null

如果想只從左表(A)中產(chǎn)生一套記錄,但不包含右表(B)的記錄,可以通過設(shè)置where語句來執(zhí)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(jù)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(jié)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(jié)果是兩個(gè)表的乘積,即笛卡爾積

笛卡爾(Descartes)乘積又叫直積。假設(shè)集合A={a,b},集合B={0,1,2},則兩個(gè)集合的笛卡爾積為{(a,0),(a,1),(a,2),(b,0),(b,1), (b,2)}??梢詳U(kuò)展到多個(gè)集合的情況。類似的例子有,如果A表示某學(xué)校學(xué)生的集合,B表示該學(xué)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(zhí)行:mysql> select * from A inner join B; 試一試

#在執(zhí)行mysql> select * from A cross join B on A.name = B.name; 試一試

實(shí)際上,在 MySQL 中(僅限于 MySQL) CROSS JOIN 與 INNER JOIN 的表現(xiàn)是一樣的,在不指定 ON 條件得到的結(jié)果都是笛卡爾積,反之取得兩個(gè)表完全匹配的結(jié)果。 INNER JOIN 與 CROSS JOIN 可以省略 INNER 或 CROSS 關(guān)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(chǎn)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(yōu)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(shù)據(jù)庫中比較(10w數(shù)據(jù))得之,它們用時(shí)幾乎相同,第一個(gè)是顯示的inner join,后一個(gè)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(shí),應(yīng)該清楚的知道以下幾點(diǎn):

(1). on與 where的執(zhí)行順序

ON 條件(“A LEFT JOIN B ON 條件表達(dá)式”中的ON)用來決定如何從 B 表中檢索數(shù)據(jù)行。如果 B 表中沒有任何一行數(shù)據(jù)匹配 ON 的條件,將會額外生成一行所有列為 NULL 的數(shù)據(jù),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(chǎn)生的數(shù)據(jù)中檢索過濾。

所以我們要注意:在使用Left (right) join的時(shí)候,一定要在先給出盡可能多的匹配滿足條件,減少Where的執(zhí)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(zhí)行性能來看第二個(gè)顯然更加省時(shí)。

(2).注意ON 子句和 WHERE 子句的不同

如作者舉了一個(gè)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(shù)據(jù)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(shù)據(jù)中過濾掉不符合條件的數(shù)據(jù)行。

(3).盡量避免子查詢,而用join

往往性能這玩意兒,更多時(shí)候體現(xiàn)在數(shù)據(jù)量比較大的時(shí)候,此時(shí),我們應(yīng)該避免復(fù)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;

聲明:本網(wǎng)頁內(nèi)容旨在傳播知識,若有侵權(quán)等問題請及時(shí)與本網(wǎng)聯(lián)系,我們將在第一時(shí)間刪除處理。TEL:177 7030 7066 E-MAIL:11247931@qq.com

文檔

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(lián)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
推薦度:
標(biāo)簽: 連接 mysql study
  • 熱門焦點(diǎn)

最新推薦

猜你喜歡

熱門推薦

專題
Top