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Mysql-高级查询语句

2022-06-05 20:42 4375 查看

Mysql-高级查询语句

本博客中数据来源于:mysql官方练习数据库

该数据库结构为:

1.控制语句

1.1DISTINCT

distinct用于记录该字段有多少不同的字段。如下所示,title表中第二个字段title有多条重复记录。

使用DISTINCT语句可以方便查询该字段有多少不同的值。

mysql> SELECT distinct title FROM `titles`;
+--------------------+
| title              |
+--------------------+
| Assistant Engineer |
| Senior Staff       |
| Engineer           |
| Staff              |
| Technique Leader   |
| Senior Engineer    |
| Manager            |
+--------------------+
7 rows in set (0.18 sec)

配合count使用,可以计算该列不同值的个数。

mysql> SELECT count(distinct title) FROM `titles`;
+-----------------------+
| count(distinct title) |
+-----------------------+
|                     7 |
+-----------------------+
1 row in set (0.09 sec)

1.2 WHERE

使用where语句可以查看限定条件下的数据。

employees表中记录不同员工的雇佣日期。

通过where语句可以筛选出所有雇佣日期为1986-06-26的雇员

mysql> select * from employees where hire_date = '1986-06-26';
+--------+------------+----------------+-----------------+--------+------------+
| emp_no | birth_date | first_name     | last_name       | gender | hire_date  |
+--------+------------+----------------+-----------------+--------+------------+
|  10001 | 1953-09-02 | Georgi         | Facello         | M      | 1986-06-26 |
|  21891 | 1960-08-25 | Shuzo          | Bernatsky       | M      | 1986-06-26 |
|  27425 | 1958-11-06 | Maik           | Gunderson       | M      | 1986-06-26 |
|  29250 | 1952-08-11 | Radoslaw       | Besancenot      | M      | 1986-06-26 |
|  32297 | 1955-03-24 | Chandrasekaran | Loncour         | M      | 1986-06-26 |
|  35301 | 1956-01-31 | Basim          | Worfolk         | F      | 1986-06-26 |
|  35565 | 1957-12-04 | Chiradeep      | Yeung           | F      | 1986-06-26 |
|  41692 | 1961-08-08 | True           | Tokunaga        | M      | 1986-06-26 |
————————————————————————————————————————————————————————————————————————————————
| 478432 | 1962-08-10 | Florian        | Beeson          | F      | 1986-06-26 |
| 478983 | 1958-08-21 | Hugo           | Litvinov        | F      | 1986-06-26 |
| 483397 | 1956-05-05 | Xuedong        | Vanwelkenhuysen | M      | 1986-06-26 |
| 499666 | 1955-12-03 | Martine        | Pollock         | M      | 1986-06-26 |
+--------+------------+----------------+-----------------+--------+------------+
83 rows in set (0.07 sec)

配合and语句可以筛选出雇佣日期为1986-06-26且性别(gender)为F(女性)的雇员。

mysql> select * from employees where hire_date = '1986-06-26' AND gender = 'F';
+--------+------------+------------+-------------+--------+------------+
| emp_no | birth_date | first_name | last_name   | gender | hire_date  |
+--------+------------+------------+-------------+--------+------------+
|  35301 | 1956-01-31 | Basim      | Worfolk     | F      | 1986-06-26 |
|  35565 | 1957-12-04 | Chiradeep  | Yeung       | F      | 1986-06-26 |
|  50733 | 1952-06-07 | Hironoby   | Nivat       | F      | 1986-06-26 |
|  52077 | 1964-12-14 | Rosella    | Streit      | F      | 1986-06-26 |
————————————————————————————————————————————————————————————————————————
| 458304 | 1963-12-01 | Shuzo      | Kirkerud    | F      | 1986-06-26 |
| 464413 | 1964-02-16 | Mart       | Lorho       | F      | 1986-06-26 |
| 478432 | 1962-08-10 | Florian    | Beeson      | F      | 1986-06-26 |
| 478983 | 1958-08-21 | Hugo       | Litvinov    | F      | 1986-06-26 |
+--------+------------+------------+-------------+--------+------------+
35 rows in set (0.06 sec)

配和or语句可以筛选两个条件符合其中一个的记录。如雇佣日期为1986-06-26或1986-09-17的雇员。

mysql> select * from employees where hire_date = '1986-06-26' or hire_date = '1986-09-17';
+--------+------------+----------------+-----------------+--------+------------+
| emp_no | birth_date | first_name     | last_name       | gender | hire_date  |
+--------+------------+----------------+-----------------+--------+------------+
|  10001 | 1953-09-02 | Georgi         | Facello         | M      | 1986-06-26 |
|  17761 | 1958-04-19 | Adin           | Matheson        | F      | 1986-09-17 |
|  21891 | 1960-08-25 | Shuzo          | Bernatsky       | M      | 1986-06-26 |
|  23606 | 1956-06-15 | Theirry        | Zirintsis       | M      | 1986-09-17 |
|  23680 | 1962-07-14 | Mary           | Navazio         | F      | 1986-09-17 |
————————————————————————————————————————————————————————————————————————
|  28104 | 1953-08-12 | Dmitry         | Marsiglia       | F      | 1986-09-17 |
|  29250 | 1952-08-11 | Radoslaw       | Besancenot      | M      | 1986-06-26 |
|  29615 | 1958-12-30 | Atreye         | Conry           | M      | 1986-09-17 |
————————————————————————————————————————————————————————————————————————
| 499666 | 1955-12-03 | Martine        | Pollock         | M      | 1986-06-26 |
+--------+------------+----------------+-----------------+--------+------------+
175 rows in set (0.06 sec)

当需要多个条件同时筛选,可以通过括号将需要同时筛选的条件括起来。下面这个例子则是将雇佣日期为'1986-06-26'且性别为F的员工或雇佣日期为1986-09-17'且性别为M的员工都筛选出来。

mysql> select * from employees where (hire_date = '1986-06-26' AND gender = 'F') OR (hire_date= '1986-09-17' AND gender = 'M');
+--------+------------+-------------+-------------+--------+------------+
| emp_no | birth_date | first_name  | last_name   | gender | hire_date  |
+--------+------------+-------------+-------------+--------+------------+
|  23606 | 1956-06-15 | Theirry     | Zirintsis   | M      | 1986-09-17 |
|  27972 | 1955-07-22 | Chaosheng   | Stanger     | M      | 1986-09-17 |
|  29615 | 1958-12-30 | Atreye      | Conry       | M      | 1986-09-17 |
|  32120 | 1962-12-16 | Mingdong    | Byoun       | M      | 1986-09-17 |
|  35301 | 1956-01-31 | Basim       | Worfolk     | F      | 1986-06-26 |
|  35565 | 1957-12-04 | Chiradeep   | Yeung       | F      | 1986-06-26 |
|  37675 | 1955-10-01 | Chikara     | Marshall    | M      | 1986-09-17 |
——————————————————————————————————————————————————————————————————————————
| 475938 | 1965-01-14 | Shigeaki    | Lanteri     | M      | 1986-09-17 |
| 478432 | 1962-08-10 | Florian     | Beeson      | F      | 1986-06-26 |
| 478641 | 1964-05-05 | Pramod      | Litvinov    | M      | 1986-09-17 |
| 478983 | 1958-08-21 | Hugo        | Litvinov    | F      | 1986-06-26 |
| 481538 | 1963-07-24 | Percy       | Szmurlo     | M      | 1986-09-17 |
| 482602 | 1954-06-18 | Aamer       | Lagarias    | M      | 1986-09-17 |
| 485137 | 1960-12-09 | Bingning    | Krohn       | M      | 1986-09-17 |
| 488143 | 1964-10-26 | Constantine | McClure     | M      | 1986-09-17 |
| 490587 | 1955-10-22 | Janalee     | VanScheik   | M      | 1986-09-17 |
| 496210 | 1960-07-02 | Alagu       | Szmurlo     | M      | 1986-09-17 |
+--------+------------+-------------+-------------+--------+------------+
86 rows in set (0.07 sec)

1.3 通配符与like

通过通配符与like可以模糊查询符合需求的记录。如下面这个查询语句可以查找出所有first_name为Chao的员工。

mysql> select * from employees where first_name like 'Chao%';
+--------+------------+------------+------------------+--------+------------+
| emp_no | birth_date | first_name | last_name        | gender | hire_date  |
+--------+------------+------------+------------------+--------+------------+
|  11829 | 1952-08-01 | Chaosheng  | Pettis           | F      | 1999-02-17 |
|  12923 | 1953-09-05 | Chaosheng  | Journel          | F      | 1990-01-02 |
|  13524 | 1955-11-06 | Chaoyi     | Porotnikoff      | F      | 1985-07-27 |
|  15343 | 1955-05-21 | Chaosheng  | Luce             | F      | 1985-03-12 |
|  16019 | 1955-11-30 | Chaoyi     | Heystek          | F      | 1985-08-01 |
——————————————————————————————————————————————————————————————————————————
| 497563 | 1957-10-29 | Chaoyi     | Manders          | M      | 1990-12-02 |
| 497796 | 1953-10-15 | Chaoyi     | Kushnir          | M      | 1988-07-19 |
| 498761 | 1958-10-09 | Chaoyi     | Kaiserswerth     | M      | 1986-01-01 |
| 498894 | 1955-02-10 | Chaoyi     | Bahr             | F      | 1986-09-21 |
| 499450 | 1963-04-23 | Chaosheng  | Pargaonkar       | F      | 1988-07-13 |
| 499876 | 1959-03-19 | Chaosheng  | Orlowska         | M      | 1988-02-20 |
+--------+------------+------------+------------------+--------+------------+
452 rows in set (0.05 sec)

百分号%代表零或多个字符,而下划线_代表一个字符。

常见的通配符使用方法如下。

  • A_Z :A与Z之间有一个字符
  • ABC% :以ABC开头的字符串后有零或多个字符
  • %XYZ :以XYZ结尾的字符串,前有零或多个字符
  • %AN%:AN的前后有零或多个字符
  • _AN%:AN前有一个字符,AN后有零或多个字符

1.4 ORDER BY

ORDER BY 用于对字段的排序,默认是升序。如使用order by语句对员工的生日进行升序排序。(出生早的在前面)

mysql> select * from employees order by birth_date limit 100;
+--------+------------+------------+--------------+--------+------------+
| emp_no | birth_date | first_name | last_name    | gender | hire_date  |
+--------+------------+------------+--------------+--------+------------+
| 406121 | 1952-02-01 | Supot      | Remmele      | M      | 1989-01-27 |
|  91374 | 1952-02-01 | Eishiro    | Kuzuoka      | M      | 1992-02-12 |
——————————————————————————————————————————————————————————————————————————
|  87461 | 1952-02-01 | Moni       | Decaestecker | M      | 1986-10-06 |
| 207658 | 1952-02-01 | Kiyokazu   | Whitcomb     | M      | 1988-07-26 |
| 498106 | 1952-02-02 | Shai       | Calkin       | M      | 1990-12-02 |
| 405962 | 1952-02-02 | Moto       | Staudhammer  | M      | 1995-02-20 |
| 409040 | 1952-02-02 | Mayuko     | Pardalos     | M      | 1991-07-03 |
|  94615 | 1952-02-02 | Toshimi    | Bruckman     | M      | 1989-04-29 |
|  93928 | 1952-02-02 | Tomoyuki   | Axelband     | F      | 1987-09-08 |
| 244265 | 1952-02-02 | Pantung    | Halevi       | M      | 1993-11-12 |
——————————————————————————————————————————————————————————————————————————
| 239891 | 1952-02-02 | Percy      | Tischendorf  | M      | 1986-11-10 |
| 455745 | 1952-02-02 | Moni       | Giveon       | F      | 1994-01-17 |
| 462774 | 1952-02-02 | Moie       | Chinal       | F      | 1997-09-20 |
| 203054 | 1952-02-02 | Hausi      | Krohn        | M      | 1994-11-23 |
| 463921 | 1952-02-02 | Lalit      | Bahr         | M      | 1987-05-27 |
| 469911 | 1952-02-02 | Mizuhito   | Szemeredi    | F      | 1990-02-24 |
| 107344 | 1952-02-02 | Kiyomitsu  | Gelosh       | F      | 1989-12-03 |
| 206240 | 1952-02-03 | Oksana     | Zykh         | M      | 1988-03-22 |
| 406833 | 1952-02-03 | Aiichiro   | Kobuchi      | M      | 1990-11-23 |
|  70603 | 1952-02-03 | Otilia     | Parhami      | M      | 1991-01-28 |
|  61129 | 1952-02-03 | Rildo      | Bain         | M      | 1992-04-03 |
——————————————————————————————————————————————————————————————————————————
|  93975 | 1952-02-03 | Gererd     | Calkin       | M      | 1988-04-18 |
| 221356 | 1952-02-03 | Pradeep    | Hagimont     | M      | 1987-09-24 |
| 294298 | 1952-02-03 | Vesna      | Coney        | M      | 1999-06-28 |
+--------+------------+------------+--------------+--------+------------+
100 rows in set (0.07 sec)

使用DESC则为倒序排序。

mysql> select * from employees order by birth_date DESC limit 100;
+--------+------------+----------------+-----------------+--------+------------+
| emp_no | birth_date | first_name     | last_name       | gender | hire_date  |
+--------+------------+----------------+-----------------+--------+------------+
| 457832 | 1965-02-01 | Jackson        | Zielinski       | F      | 1985-04-04 |
| 424584 | 1965-02-01 | Kagan          | Dredge          | M      | 1994-02-26 |
——————————————————————————————————————————————————————————————————————————
| 284045 | 1965-02-01 | Fun            | Seiwald         | M      | 1994-06-24 |
| 294996 | 1965-02-01 | Tomoyuki       | Vigier          | F      | 1991-07-08 |
| 437369 | 1965-02-01 | Kazuhide       | Biran           | F      | 1995-01-25 |
|  93278 | 1965-02-01 | Magdalena      | Penn            | F      | 1987-04-27 |
| 418860 | 1965-02-01 | Ymte           | Dalton          | F      | 1992-11-22 |
| 109598 | 1965-02-01 | Stamatina      | Auyong          | M      | 1988-07-05 |
|  40785 | 1965-01-31 | Youpyo         | Castellani      | M      | 1998-09-04 |
|  97661 | 1965-01-31 | Avishai        | Hegner          | M      | 1993-05-08 |
——————————————————————————————————————————————————————————————————————————
| 235269 | 1965-01-31 | Keiichiro      | Feldmann        | F      | 1989-06-28 |
|  37719 | 1965-01-31 | Nakhoon        | Buescher        | M      | 1986-07-28 |
|  52770 | 1965-01-31 | Shugo          | Yurek           | M      | 1986-01-06 |
+--------+------------+----------------+-----------------+--------+------------+
100 rows in set (0.07 sec)

2.函数

2.1 数学函数

数学函数主要是对数字的处理。

现有这样一张表(自建)

mysql> select * from test_math;
+------+------+------+
| num1 | num2 | num3 |
+------+------+------+
|   15 |   20 |   30 |
|   17 |   10 |    3 |
|   -2 |   10 |   25 |
+------+------+------+
3 rows in set (0.01 sec)

函数**abs()**返回该字段的绝对值。

mysql> select abs(num1) from test_math;
+-----------+
| abs(num1) |
+-----------+
|        15 |
|        17 |
|         2 |
+-----------+
3 rows in set (0.00 sec)

函数**rand ()**返回0-1的随机值。

mysql> select rand();
+--------------------+
| rand()             |
+--------------------+
| 0.5209844825053443 |
+--------------------+
1 row in set (0.00 sec)

函数**mod(x,y)**返回x/y的余数。

mysql> select num1,num2,mod(num1,num2) from test_math;
+------+------+----------------+
| num1 | num2 | mod(num1,num2) |
+------+------+----------------+
|   15 |   20 |             15 |
|   17 |   10 |              7 |
|   -2 |   10 |             -2 |
+------+------+----------------+
3 rows in set (0.00 sec)

函数**power (x,y)**返回x的y次方(第一行e23,代表10的23次方,由于数字过大,采用了科学计数法)

mysql> select num1,num2,power(num1,num2) from test_math;
+------+------+-----------------------+
| num1 | num2 | power(num1,num2)      |
+------+------+-----------------------+
|   15 |   20 | 3.3252567300796506e23 |
|   17 |   10 |         2015993900449 |
|   -2 |   10 |                  1024 |
+------+------+-----------------------+
3 rows in set (0.00 sec)

函数**greatest (xl,x2. . .)**返回集合中的最大数。

mysql> select num1,num2,num3,greatest(num1,num2,num3) from test_math;
+------+------+------+--------------------------+
| num1 | num2 | num3 | greatest(num1,num2,num3) |
+------+------+------+--------------------------+
|   15 |   20 |   30 |                       30 |
|   17 |   10 |    3 |                       17 |
|   -2 |   10 |   25 |                       25 |
+------+------+------+--------------------------+
3 rows in set (0.00 sec)

函数**least (xl,x2. . .)**返回集合中的最小数。

mysql> select num1,num2,num3,least(num1,num2,num3) from test_math;
+------+------+------+-----------------------+
| num1 | num2 | num3 | least(num1,num2,num3) |
+------+------+------+-----------------------+
|   15 |   20 |   30 |                    15 |
|   17 |   10 |    3 |                     3 |
|   -2 |   10 |   25 |                    -2 |
+------+------+------+-----------------------+
3 rows in set (0.00 sec)

其余常用的数学函数还有:

round (X)

返回离x最近的整数

round(x, y)

保留x的y位小数四舍五入后的值

sqrt(x)

返回x的平方根

truncate (x, y)

返回数字x截断为y位小数的值

ceil (x)

返回大于或等于x的最小整数

floor (x)

返回小于或等于x的最大整数

2.2 聚合函数

主要涉及对字段数据的处理,依旧使用test_math进行演示。

mysql> select * from test_math;
+------+------+------+
| num1 | num2 | num3 |
+------+------+------+
|   15 |   20 |   30 |
|   17 |   10 |    3 |
|   -2 |   10 |   25 |
+------+------+------+
3 rows in set (0.01 sec)

函数**avg ()**求指定列的平均值

mysql> select AVG(num3) from test_math;
+-----------+
| AVG(num3) |
+-----------+
|   19.3333 |
+-----------+
1 row in set (0.00 sec)

函数**min()**求指定列的最小值

mysql> select min(num3) from test_math;
+-----------+
| min(num3) |
+-----------+
|         3 |
+-----------+
1 row in set (0.00 sec)

函数**MAX()**求指定列的最大值

mysql> select max(num3) from test_math;
+-----------+
| max(num3) |
+-----------+
|        30 |
+-----------+
1 row in set (0.00 sec)

函数**sum (x)**返回指定列的和

mysql> select sum(num3) from test_math;
+-----------+
| sum(num3) |
+-----------+
|        58 |
+-----------+
1 row in set (0.00 sec)

2.3 字符串函数

字符串函数顾名思义就是针对字符串进行处理的函数

现有表

mysql> select * from test_char;
+-------+-------+----------------+
| char1 | char2 | char3          |
+-------+-------+----------------+
| KY    | 02    | zengguoqiang   |
| KY    | 11    | changzeyou     |
| CT    | 83    | wudifenghuolun |
+-------+-------+----------------+
3 rows in set (0.00 sec)

函数**concat (x, y)**将x和y拼接为一个字符,xy可以是字段,也可以是符号

mysql> select char1,char2,concat(char1,char2),concat('2022-',char1) from test_char;
+-------+-------+---------------------+-----------------------+
| char1 | char2 | concat(char1,char2) | concat('2022-',char1) |
+-------+-------+---------------------+-----------------------+
| KY    | 02    | KY02                | 2022-KY               |
| KY    | 11    | KY11                | 2022-KY               |
| CT    | 83    | CT83                | 2022-CT               |
+-------+-------+---------------------+-----------------------+
3 rows in set (0.00 sec)

函数**substr (x,y, z)**用于截取字符串,x代表字段,y代表截取位置,z代表截取长度。

mysql> select char3,substr(char3,2,4) from test_char;
+----------------+-------------------+
| char3          | substr(char3,2,4) |
+----------------+-------------------+
| zengguoqiang   | engg              |
| changzeyou     | hang              |
| wudifenghuolun | udif              |
+----------------+-------------------+
3 rows in set (0.00 sec)

函数**length (x)**可以获取字符串长度。

mysql> select char3,length(char3) from test_char;
+----------------+---------------+
| char3          | length(char3) |
+----------------+---------------+
| zengguoqiang   |            12 |
| changzeyou     |            10 |
| wudifenghuolun |            14 |
+----------------+---------------+
3 rows in set (0.00 sec)

函数**replace(x, y, z)**用于替换字符串内容,x代表被替换的字符串,y代表被替换的内容,z代表替换的内容。这三个可以是字符串也可是字段名称。

mysql> select *,replace(char3,'uo',char2) from test_char;
+-------+-------+----------------+---------------------------+
| char1 | char2 | char3          | replace(char3,'uo',char2) |
+-------+-------+----------------+---------------------------+
| KY    | 02    | zengguoqiang   | zengg02qiang              |
| KY    | 11    | changzeyou     | changzeyou                |
| CT    | 83    | wudifenghuolun | wudifengh83lun            |
+-------+-------+----------------+---------------------------+
3 rows in set (0.00 sec

其余常用字符串函数还有:

upper (x)

将字符串x的所有字母变成大写字母

lower (x)

将字符串x的所有字母变成小写字母

left(x,y)

返回字符串x的前y个字符

right (x,y)

返回字符串x的后y个字符

repeat (x, y)

将字符串x重复y次

space (x)

返回x个空格

3. 连接(join)

mysql在涉及多表关联字段查询的时候可以通过join来实现指定内容的查找,mysql连接种类大致分为:

  • INNER JOIN(内连接,或等值连接):获取两个表中字段匹配关系的记录。
  • **LEFT JOIN(左连接):**获取左表所有记录,即使右表没有对应匹配的记录。
  • RIGHT JOIN(右连接): 与 LEFT JOIN 相反,用于获取右表所有记录,即使左表没有对应匹配的记录。

现有两表

mysql> select * from LEFT_tb1;
+--------+----------+
| ssh_id | ssh_name |
+--------+----------+
|      1 | zhangsan |
|      2 | lisi     |
|      3 | yefan    |
|      7 | admin    |
+--------+----------+
4 rows in set (0.00 sec)

mysql> select * from RIGHT_tb;
+--------+-----------+---------------+------------+
| ssh_id | ssh_count | ssh_ip        | last_log   |
+--------+-----------+---------------+------------+
|      1 |        20 | 192.168.80.30 | 2022-06-04 |
|      2 |        33 | 192.168.80.50 | 2022-05-10 |
|      3 |        45 | 192.168.80.20 | 2022-05-25 |
|      4 |        12 | 192.168.80.11 | 2022-05-03 |
|      5 |         1 | 100.0.0.1     | 2022-04-26 |
+--------+-----------+---------------+------------+
5 rows in set (0.00 sec)

3.1 INNER JOIN(内连接)

通过INNER JOIN可以查看表LEFT_tb1与RIGHT_tb关联字段ssh_id的对应记录。

mysql> select l.ssh_id,l.ssh_name,r.ssh_count,r.ssh_ip,r.last_log from LEFT_tb1 l INNER JOIN RIGHT_tb r on l.ssh_id = r.ssh_id;
+--------+----------+-----------+---------------+------------+
| ssh_id | ssh_name | ssh_count | ssh_ip        | last_log   |
+--------+----------+-----------+---------------+------------+
|      1 | zhangsan |        20 | 192.168.80.30 | 2022-06-04 |
|      2 | lisi     |        33 | 192.168.80.50 | 2022-05-10 |
|      3 | yefan    |        45 | 192.168.80.20 | 2022-05-25 |
+--------+----------+-----------+---------------+------------+
3 rows in set (0.00 sec)

INNER JOIN关系类似于上图,只获取交叉数据。

3.2 LEFT JOIN(左连接)

MySQL left join 与 join 有所不同。 MySQL LEFT JOIN 会读取左边数据表的全部数据,即便右边表无对应数据。无对应数据字段用NULL填充。

mysql> select l.ssh_id,l.ssh_name,r.ssh_count,r.ssh_ip,r.last_log from LEFT_tb1 l LEFT JOIN RIGHT_tb r on l.ssh_iid = r.ssh_id;
+--------+----------+-----------+---------------+------------+
| ssh_id | ssh_name | ssh_count | ssh_ip        | last_log   |
+--------+----------+-----------+---------------+------------+
|      1 | zhangsan |        20 | 192.168.80.30 | 2022-06-04 |
|      2 | lisi     |        33 | 192.168.80.50 | 2022-05-10 |
|      3 | yefan    |        45 | 192.168.80.20 | 2022-05-25 |
|      7 | admin    |      NULL | NULL          | NULL       |
+--------+----------+-----------+---------------+------------+
4 rows in set (0.00 sec)

3.3 RIGHT JOIN(右连接)

MySQL RIGHT JOIN 会读取右边数据表的全部数据,即便左边边表无对应数据。

mysql> select l.ssh_id,l.ssh_name,r.ssh_count,r.ssh_ip,r.last_log from LEFT_tb1 l RIGHT JOIN RIGHT_tb r on l.ssh__id = r.ssh_id;
+--------+----------+-----------+---------------+------------+
| ssh_id | ssh_name | ssh_count | ssh_ip        | last_log   |
+--------+----------+-----------+---------------+------------+
|      1 | zhangsan |        20 | 192.168.80.30 | 2022-06-04 |
|      2 | lisi     |        33 | 192.168.80.50 | 2022-05-10 |
|      3 | yefan    |        45 | 192.168.80.20 | 2022-05-25 |
|   NULL | NULL     |        12 | 192.168.80.11 | 2022-05-03 |
|   NULL | NULL     |         1 | 100.0.0.1     | 2022-04-26 |
+--------+----------+-----------+---------------+------------+
5 rows in set (0.00 sec)

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