1.条件查询：
语法：
#（1）（2）（3） 代表执行步骤。
select
查询列表 （3）
from
表名 （1）
where
筛选条件；（2）
2.筛选条件的分类
一. 按条件表达式筛选
简单条件运算符：> , <, = , >= , <= , <>(不等于）, !=，<=>(安全等于）。二. 按逻辑表达式筛选（用于连接条件表达式）
逻辑运算符：&& ， || ， ！（与，或，非）
或者是 ：and ，or,not(mysql推荐使用）-
三. 模糊查询：
- like（通常与通配符使用'%'，'_')
- between and(包含临界值）
- in（判断某一字段的值是否属于in列表中的某一项）
- is null && is not null 可以判断Null值。
注意
is null pk <=>.
is null:仅仅可以判断NULL值，可读性高。
<=>:既可以判断null值，也可以判断普通数值，可读性较低。
√一. 按条件表达式筛选的练习
# 案例1 查询月薪大于12000的员工信息；
mysql> select * from employees where salary >12000;
+-------------+------------+-----------+----------+--------------------+---------+----------+----------------+------------+---------------+---------------------+
| employee_id | first_name | last_name | email | phone_number | job_id | salary | commission_pct | manager_id | department_id | hiredate |
+-------------+------------+-----------+----------+--------------------+---------+----------+----------------+------------+---------------+---------------------+
| 100 | Steven | K_ing | SKING | 515.123.4567 | AD_PRES | 24000.00 | NULL | NULL | 90 | 1992-04-03 00:00:00 |
| 101 | Neena | Kochhar | NKOCHHAR | 515.123.4568 | AD_VP | 17000.00 | NULL | 100 | 90 | 1992-04-03 00:00:00 |
| 102 | Lex | De Haan | LDEHAAN | 515.123.4569 | AD_VP | 17000.00 | NULL | 100 | 90 | 1992-04-03 00:00:00 |
...
...
#案例2 查询部门编号不等于90的员工名和部门编号
mysql> select last_name,department_id from employees where department_id <>90;
+-------------+---------------+
| last_name | department_id |
+-------------+---------------+
| Hunold | 60 |
| Ernst | 60 |
| Austin | 60 |
| Pataballa | 60 |
| Lorentz | 60 |
| Greenberg | 100 |
| Faviet | 100 |
| Chen | 100 |
| Sciarra | 100 |
| Urman | 100 |
| Popp | 100 |
| Raphaely | 30 |
| Khoo | 30 |
| Baida | 30 |
...
...
...
103 rows in set (0.37 sec)
√二. 按逻辑表达式筛选的练习
案例一：查询工资在10000到20000之间的员工名，工资以及奖金
mysql> select last_name, salary ,commission_pct from employees where salary >=10000 AND salary <=20000;
+-----------+----------+----------------+
| last_name | salary | commission_pct |
+-----------+----------+----------------+
| Kochhar | 17000.00 | NULL |
| De Haan | 17000.00 | NULL |
| Greenberg | 12000.00 | NULL |
| Raphaely | 11000.00 | NULL |
| Russell | 14000.00 | 0.40 |
| Partners | 13500.00 | 0.30 |
| Errazuriz | 12000.00 | 0.30 |
| Cambrault | 11000.00 | 0.30 |
| Zlotkey | 10500.00 | 0.20 |
| Tucker | 10000.00 | 0.30 |
| K_ing | 10000.00 | 0.35 |
| Vishney | 10500.00 | 0.25 |
| Ozer | 11500.00 | 0.25 |
| Bloom | 10000.00 | 0.20 |
| Abel | 11000.00 | 0.30 |
| Hartstein | 13000.00 | NULL |
| Baer | 10000.00 | NULL |
| Higgins | 12000.00 | NULL |
+-----------+----------+----------------+
18 rows in set (0.00 sec)
案例2:查询部门编号不在90到110之间，工资高于15000的员工信息；
mysql> select * from employees where department_id <= 90 AND department_id >=110 or salary > 15000;
+-------------+------------+-----------+----------+--------------+---------+----------+----------------+------------+---------------+---------------------+
| employee_id | first_name | last_name | email | phone_number | job_id | salary | commission_pct | manager_id | department_id | hiredate |
+-------------+------------+-----------+----------+--------------+---------+----------+----------------+------------+---------------+---------------------+
| 100 | Steven | K_ing | SKING | 515.123.4567 | AD_PRES | 24000.00 | NULL | NULL | 90 | 1992-04-03 00:00:00 |
| 101 | Neena | Kochhar | NKOCHHAR | 515.123.4568 | AD_VP | 17000.00 | NULL | 100 | 90 | 1992-04-03 00:00:00 |
| 102 | Lex | De Haan | LDEHAAN | 515.123.4569 | AD_VP | 17000.00 | NULL | 100 | 90 | 1992-04-03 00:00:00 |
+-------------+------------+-----------+----------+--------------+---------+----------+----------------+------------+---------------+---------------------+
3 rows in set (0.01 sec)
√三. 模糊查询的练习
#√1.like案例：
案例一：查询员工名字含有a字母的员工信息。
mysql> select * from employees where last_name like'%a%';
+-------------+-------------+------------+----------+--------------------+------------+----------+----------------+------------+---------------+---------------------+
| employee_id | first_name | last_name | email | phone_number | job_id | salary | commission_pct | manager_id | department_id | hiredate |
+-------------+-------------+------------+----------+--------------------+------------+----------+----------------+------------+---------------+---------------------+
| 101 | Neena | Kochhar | NKOCHHAR | 515.123.4568 | AD_VP | 17000.00 | NULL | 100 | 90 | 1992-04-03 00:00:00 |
| 102 | Lex | De Haan | LDEHAAN | 515.123.4569 | AD_VP | 17000.00 | NULL | 100 | 90 | 1992-04-03 00:00:00 |
| 105 | David | Austin | DAUSTIN | 590.423.4569 | IT_PROG | 4800.00 | NULL | 103 | 60 | 1998-03-03 00:00:00 |
| 106 | Valli | Pataballa | VPATABAL | 590.423.4560 | IT_PROG | 4800.00 | NULL | 103 | 60 | 1998-03-03 00:00:00 |
| 109 | Daniel | Faviet | DFAVIET | 515.124.4169 | FI_ACCOUNT | 9000.00 | NULL | 108 | 100 | 1998-03-03 00:00:00 |
| 111 | Ismael | Sciarra | ISCIARRA | 515.124.4369 | FI_ACCOUNT | 7700.00 | NULL | 108 | 100 | 2000-09-09 00:00:00 |
| 112 | Jose Manuel | Urman | JMURMAN | 515.124.4469 | FI_ACCOUNT | 7800.00 | NULL | 108 | 100 | 2000-09-09 00:00:00 |
| 114 | Den | Raphaely | DRAPHEAL | 515.127.4561 | PU_MAN | 11000.00 | NULL | 100 | 30 | 2000-09-09 00:00:00 |
| 116 | Shelli | Baida | SBAIDA | 515.127.4563 | PU_CLERK | 2900.00 | NULL | 114 | 30 | 2000-09-09 00:00:00 |
| 117 | Sigal | Tobias | STOBIAS | 515.127.4564 | PU_CLERK | 2800.00 | NULL | 114 | 30 | 2000-09-09 00:00:00 |
| 119 | Karen | Colmenares | KCOLMENA | 515.127.4566 | PU_CLERK | 2500.00 | NULL | 114 | 30 | 2000-09-09 00:00:00 |
| 122 | Payam | Kaufling | PKAUFLIN | 650.123.3234 | ST_MAN | 7900.00 | NULL | 100 | 50 | 2004-02-06 00:00:00 |
...
...
案例二：查询员工名字中第三个字母为n,第五个字母为l的员工名和工资
mysql> select last_name,salary from employees where last_name like '__i_a%';
+-----------+---------+
| last_name | salary |
+-----------+---------+
| Baida | 2900.00 |
+-----------+---------+
1 row in set (0.00 sec)
案例三：查询员工名中第二个字母为_的员工名
mysql> select last_name from employees where last_name like '_$_%' ESCAPE'$';
+-----------+
| last_name |
+-----------+
| K_ing |
| K_ing |
+-----------+
2 rows in set (0.00 sec)
#√2.between and
案例一：查询编号在100到120之间的员工信息
mysql> select * from employees where department_id between 100 and 120;
+-------------+-------------+-----------+----------+--------------+------------+----------+----------------+------------+---------------+---------------------+
| employee_id | first_name | last_name | email | phone_number | job_id | salary | commission_pct | manager_id | department_id | hiredate |
+-------------+-------------+-----------+----------+--------------+------------+----------+----------------+------------+---------------+---------------------+
| 108 | Nancy | Greenberg | NGREENBE | 515.124.4569 | FI_MGR | 12000.00 | NULL | 101 | 100 | 1998-03-03 00:00:00 |
| 109 | Daniel | Faviet | DFAVIET | 515.124.4169 | FI_ACCOUNT | 9000.00 | NULL | 108 | 100 | 1998-03-03 00:00:00 |
| 110 | John | Chen | JCHEN | 515.124.4269 | FI_ACCOUNT | 8200.00 | NULL | 108 | 100 | 2000-09-09 00:00:00 |
| 111 | Ismael | Sciarra | ISCIARRA | 515.124.4369 | FI_ACCOUNT | 7700.00 | NULL | 108 | 100 | 2000-09-09 00:00:00 |
| 112 | Jose Manuel | Urman | JMURMAN | 515.124.4469 | FI_ACCOUNT | 7800.00 | NULL | 108 | 100 | 2000-09-09 00:00:00 |
| 113 | Luis | Popp | LPOPP | 515.124.4567 | FI_ACCOUNT | 6900.00 | NULL | 108 | 100 | 2000-09-09 00:00:00 |
| 205 | Shelley | Higgins | SHIGGINS | 515.123.8080 | AC_MGR | 12000.00 | NULL | 101 | 110 | 2016-03-03 00:00:00 |
| 206 | William | Gietz | WGIETZ | 515.123.8181 | AC_ACCOUNT | 8300.00 | NULL | 205 | 110 | 2016-03-03 00:00:00 |
+-------------+-------------+-----------+----------+--------------+------------+----------+----------------+------------+---------------+---------------------+
8 rows in set (0.37 sec)
mysql>
#√3.in
案例一：查询员工工种号是 IT_PROG,AD_VP,AD_PRES中的员工名和工种编号；
mysql> select last_name,job_id from employees where job_id in('IT_PROG','AD_VP','AD_PRES');//里面的字符值要用单引号引起来。
+-----------+---------+
| last_name | job_id |
+-----------+---------+
| K_ing | AD_PRES |
| Kochhar | AD_VP |
| De Haan | AD_VP |
| Hunold | IT_PROG |
| Ernst | IT_PROG |
| Austin | IT_PROG |
| Pataballa | IT_PROG |
| Lorentz | IT_PROG |
+-----------+---------+
8 rows in set (0.34 sec)
mysql>
#√4.is null
案例1:查询没有奖金的员工名和奖金率；
mysql> select last_name ,commission_pct from employees where commission_pct is null;
+-------------+----------------+
| last_name | commission_pct |
+-------------+----------------+
| K_ing | NULL |
| Kochhar | NULL |
| De Haan | NULL |
| Hunold | NULL |
| Ernst | NULL |
| Austin | NULL |
| Pataballa | NULL |
| Lorentz | NULL |
| Greenberg | NULL |
| Faviet | NULL |
| Chen | NULL |
| Sciarra | NULL |
| Urman | NULL |
| Popp | NULL |
| Raphaely | NULL |
...
...
或者用<=>也可以达到想要的效果。
mysql> select last_name ,commission_pct from employees where commission_pct <=> null;
#√is not null
案例1:查询有奖金的员工名和奖金率；
mysql> select last_name ,commission_pct from employees where commission_pct is not null;
+------------+----------------+
| last_name | commission_pct |
+------------+----------------+
| Russell | 0.40 |
| Partners | 0.30 |
| Errazuriz | 0.30 |
| Cambrault | 0.30 |
| Zlotkey | 0.20 |
| Tucker | 0.30 |
| Bernstein | 0.25 |
| Hall | 0.25 |
| Olsen | 0.20 |
| Cambrault | 0.20 |
| Tuvault | 0.15 |
| K_ing | 0.35 |
| Sully | 0.35 |
| McEwen | 0.35 |
| Smith | 0.30 |
| Doran | 0.30 |
| Sewall | 0.25 |
| Vishney | 0.25 |
| Greene | 0.15 |
| Marvins | 0.10 |
| Lee | 0.10 |
| Ande | 0.10 |
| Banda | 0.10 |
| Ozer | 0.25 |
| Bloom | 0.20 |
| Fox | 0.20 |
| Smith | 0.15 |
| Bates | 0.15 |
| Kumar | 0.10 |
| Abel | 0.30 |
| Hutton | 0.25 |
| Taylor | 0.20 |
| Livingston | 0.20 |
| Grant | 0.15 |
| Johnson | 0.10 |
+------------+----------------+
35 rows in set (0.00 sec)
√四.其它练习
查询员工的名字和部门号和年薪
mysql> select last_name, department_id, salary*12*(1+Ifnull(commission_pct,0)) as 年薪 from employees;
+-------------+---------------+-----------+
| last_name | department_id | 年薪 |
+-------------+---------------+-----------+
| K_ing | 90 | 288000.00 |
| Kochhar | 90 | 204000.00 |
| De Haan | 90 | 204000.00 |
| Hunold | 60 | 108000.00 |
| Ernst | 60 | 72000.00 |
| Austin | 60 | 57600.00 |
| Pataballa | 60 | 57600.00 |
| Lorentz | 60 | 50400.00 |
| Greenberg | 100 | 144000.00 |
| Faviet | 100 | 108000.00 |
| Chen | 100 | 98400.00 |
| Sciarra | 100 | 92400.00 |
| Urman | 100 | 93600.00 |
| Popp | 100 | 82800.00 |
| Raphaely | 30 | 132000.00 |
| Khoo | 30 | 37200.00 |
| Baida | 30 | 34800.00 |
...
...
注：这是本人的学习笔记及练习，如果有错误的地方望指出一起讨论，谢谢！