fix

blog/php-mysql/index.md

@@ -37,3 +37,5 @@
 [高级数据操作-删除数据](blog/php-mysql/sql-senior-delete.md)
 
 [高级数据操作-查询数据](blog/php-mysql/sql-senior-select.md)
+
+[查询中的运算符](blog/php-mysql/sql-operator.md)

blog/php-mysql/sql-operator.md

+# 查询中的运算符
+
+## 1、算术运算符
+
+```
++ 加
+- 减
+* 乘
+/ 除
+% 取余/取模
+```
+
+通常不在条件中使用，而是用于结果运算（select字段中）
+
+null进行任何算术运算，结果都为null
+
+除法中除数如果为0，结果为null
+
+```sql
+mysql> select 1+1, 2-1, 2*3, 6/2, 7%2, 1/0, 1/null;
++-----+-----+-----+--------+------+------+--------+
+| 1+1 | 2-1 | 2*3 | 6/2    | 7%2  | 1/0  | 1/null |
++-----+-----+-----+--------+------+------+--------+
+|   2 |   1 |   6 | 3.0000 |    1 | NULL |   NULL |
++-----+-----+-----+--------+------+------+--------+
+```
+## 2、比较运算符
+
+```
+>  大于
+>= 大于等于
+<  小于
+<= 小于等于
+=  等于 (<=>)
+<> 不等于
+```
+
+用来在条件中限定结果
+
+```sql
+select '1' = 1, 0.02 = 0;
++---------+----------+
+| '1' = 1 | 0.02 = 0 |
++---------+----------+
+|       1 |        0 |
++---------+----------+
+```
+
+- MySQL 中没有规定select必须有数据表
+- MySQL中数据会先自动转成同类型，再比较
+- MySQL中没有bool类型，1代表true, 0代表false
+
+计算闭区间
+
+```sql
+-- 条件1 需要比 条件2小
+字段 between 条件1 and 条件2
+```
+
+```sql
+mysql> select * from my_student;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  1 | 刘备   |        1 |   18 |      2 |
+|  2 | 李四   |        1 |   19 |      1 |
+|  3 | 王五   |        2 |   20 |      2 |
+|  7 | 张飞   |        2 |   21 |      1 |
+|  8 | 关羽   |        1 |   22 |      2 |
++----+--------+----------+------+--------+
+
+-- 查找年龄在[19, 21]区间的学生
+mysql> select * from my_student where age between  19 and 21;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  2 | 李四   |        1 |   19 |      1 |
+|  3 | 王五   |        2 |   20 |      2 |
+|  7 | 张飞   |        2 |   21 |      1 |
++----+--------+----------+------+--------+
+```
+
+## 3、逻辑运算符
+
+```
+and 逻辑与
+or 逻辑或
+not 逻辑非
+```
+
+```sql
+-- 查找年龄在[19, 21]区间的学生
+select * from my_student where age >= 19 and age <= 21;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  2 | 李四   |        1 |   19 |      1 |
+|  3 | 王五   |        2 |   20 |      2 |
+|  7 | 张飞   |        2 |   21 |      1 |
++----+--------+----------+------+--------+
+
+-- 查找年龄大于20或者是男性的学生
+select * from my_student where age > 20 or gender = 1;
+mysql> select * from my_student where age > 20 or gender = 1;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  2 | 李四   |        1 |   19 |      1 |
+|  7 | 张飞   |        2 |   21 |      1 |
+|  8 | 关羽   |        1 |   22 |      2 |
++----+--------+----------+------+--------+
+```
+
+## 4、in运算符
+
+用来替代等号，判断集合
+
+基本语法
+
+```
+in (数值1, 数值2...)
+```
+
+```sql
+-- 按照学号查找学生
+mysql> select * from my_student where id in (1, 3);
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  1 | 刘备   |        1 |   18 |      2 |
+|  3 | 王五   |        2 |   20 |      2 |
++----+--------+----------+------+--------+
+```
+
+## 5、is运算符
+
+判断字段值是否为null
+
+基本语法
+```sql
+is null
+is not null
+```
+
+
+```sql
+mysql> select * from my_student;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  1 | 刘备   |        1 |   18 |      2 |
+|  2 | 李四   |        1 |   19 |      1 |
+|  3 | 王五   |        2 |   20 |      2 |
+|  7 | 张飞   |        2 |   21 |      1 |
+|  8 | 关羽   |        1 |   22 |      2 |
+|  9 | 曹操   |        1 |   20 |   NULL |
++----+--------+----------+------+--------+
+
+-- 查询为null的数据
+mysql> select * from my_student where gender is null;
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  9 | 曹操   |        1 |   20 |   NULL |
++----+--------+----------+------+--------+
+
+```
+
+## 6、like运算符
+
+模糊匹配
+
+
+like 匹配模式
+
+占位符：
+- 下划线`_` 匹配单个字符
+- 百分号`%` 匹配多个字符
+
+```sql
+mysql> select * from my_student where name like '曹_';
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  9 | 曹操   |        1 |   20 |   NULL |
++----+--------+----------+------+--------+
+
+mysql> select * from my_student where name like '曹%';
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  9 | 曹操   |        1 |   20 |   NULL |
++----+--------+----------+------+--------+
+```
+
+https://www.bilibili.com/video/BV1Vx411g7uJ?p=40&spm_id_from=pageDriver
+
+
+

blog/php-mysql/sql-senior-select.md

@@ -505,5 +505,3 @@ mysql> select * from my_student limit 4, 2;
 |  8 | 关羽   |        1 |   22 |      2 |
 +----+--------+----------+------+--------+
 ```
-
-https://www.bilibili.com/video/BV1Vx411g7uJ?p=39&spm_id_from=pageDriver
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