點(diǎn)擊下載
本文文檔

當(dāng)前位置：首頁(yè) - 科技 - 知識(shí)百科 - 正文

MySQLStudy之--MySQL表連接_MySQL

來(lái)源：懂視網(wǎng) 責(zé)編：小采時(shí)間：2020-11-09 20:13:37

MySQLStudy之--MySQL表連接_MySQL

MySQLStudy之--MySQL表連接_MySQL:MySQL Study之--MySQL 表連接一．Join語(yǔ)法概述 join 用于多表中字段之間的聯(lián)系，語(yǔ)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：

點(diǎn)擊下載本文 文檔為doc格式

導(dǎo)讀MySQLStudy之--MySQL表連接_MySQL:MySQL Study之--MySQL 表連接一．Join語(yǔ)法概述 join 用于多表中字段之間的聯(lián)系，語(yǔ)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(yǔ)法概述

join 用于多表中字段之間的聯(lián)系，語(yǔ)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(wú)對(duì)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(wú)匹配對(duì)應(yīng)記錄。

注意：mysql不支持Full join,不過可以通過UNION 關(guān)鍵字來(lái)合并 LEFT JOIN 與 RIGHT JOIN來(lái)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(wú)對(duì)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(duì)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(wú)匹配對(duì)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(yǔ)法以及性能優(yōu)化

在講MySQL的Join語(yǔ)法前還是先回顧一下聯(lián)結(jié)的語(yǔ)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(jiǎn)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(huì)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(huì)混淆

了。

一．Join語(yǔ)法概述

join 用于多表中字段之間的聯(lián)系，語(yǔ)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(wú)對(duì)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(wú)匹配對(duì)應(yīng)記錄。

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

接下來(lái)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(yǔ)句來(lái)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)}。可以擴(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(duì)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;

select a.*, b.*

from table a, table b

where a.id = b.id;

我在數(shù)據(jù)庫(kù)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(lái)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(huì)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(huì)被使用。僅在匹配階段完成以后，WHERE 子句條件才會(huì)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(lái)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(jiǎn)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(yè)內(nèi)容旨在傳播知識(shí)，若有侵權(quán)等問題請(qǐng)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