fix

blog/php-mysql/index.md

@@ -39,3 +39,7 @@
 [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)
+
+[联合查询 union](blog/php-mysql/sql-union.md)
+
+[连接查询 join](blog/php-mysql/sql-join.md)

blog/php-mysql/sql-join.md

+# 连接查询
+
+将多张表连到一起查询 导致记录行数和字段列发生变化
+
+利用一对一、一对多和多对多关系保证数据完整性
+
+## 1、连接查询的分类
+
+- 交叉连接
+- 内连接
+- 外连接
+    - 左外链接（左连接）
+    - 右外连接（右连接）
+- 自然连接
+
+
+https://www.bilibili.com/video/BV1Vx411g7uJ?p=42&spm_id_from=pageDriver
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blog/php-mysql/sql-operator.md

@@ -191,9 +191,4 @@ mysql> select * from my_student where name like '曹%';
 +----+--------+----------+------+--------+
 |  9 | 曹操   |        1 |   20 |   NULL |
 +----+--------+----------+------+--------+
-```
-
-https://www.bilibili.com/video/BV1Vx411g7uJ?p=40&spm_id_from=pageDriver
-
-
-
+```
\ No newline at end of file

blog/php-mysql/sql-union.md

+# 联合查询
+
+将多个查询结果的结果集合并到一起（纵向合并），字段数不变，多个查询结果的记录数合并
+
+## 1、应用场景
+
+1. 同一张表中不同结果合并到一起展示：男生升高升序，女生升高降序
+2. 数据量较大的表，进行分表操作，将每张表的数据合并起来显示
+
+## 2、基本语法
+
+```sql
+select 语句
+union [union 选项]
+select 语句;
+```
+
+union 选项 和select 选项基本一致
+
+- distinct 去重，默认
+- all 保存所有结果
+
+```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 |
++----+--------+----------+------+--------+
+
+-- 默认选项：distinct
+select * from my_student
+union
+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 |
++----+--------+----------+------+--------+
+
+
+select * from my_student
+union all
+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 |
+|  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 |
++----+--------+----------+------+--------+
+
+-- 只需要保证字段数量一样，不需要每次拿到的数据类型都一样
+-- 只保留第一个select的字段名
+select id, name, age from my_student
+union all
+select name, id, age  from my_student;
++--------+--------+------+
+| id     | name   | age  |
++--------+--------+------+
+| 1      | 刘备   |   18 |
+| 2      | 李四   |   19 |
+| 3      | 王五   |   20 |
+| 7      | 张飞   |   21 |
+| 8      | 关羽   |   22 |
+| 9      | 曹操   |   20 |
+| 刘备   | 1      |   18 |
+| 李四   | 2      |   19 |
+| 王五   | 3      |   20 |
+| 张飞   | 7      |   21 |
+| 关羽   | 8      |   22 |
+| 曹操   | 9      |   20 |
++--------+--------+------+
+```
+
+## 3、order by的使用
+
+联合查询中，使用order by, select语句必须使用括号
+
+```sql
+(select * from my_student where gender = 1 order by age desc)
+union
+(select * from my_student where gender = 2 order by age asc);
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  2 | 李四   |        1 |   19 |      1 |
+|  7 | 张飞   |        2 |   21 |      1 |
+|  1 | 刘备   |        1 |   18 |      2 |
+|  3 | 王五   |        2 |   20 |      2 |
+|  8 | 关羽   |        1 |   22 |      2 |
++----+--------+----------+------+--------+
+
+-- order by 要生效，必须使用limit 通常大于表的记录数
+(select * from my_student where gender = 1 order by age desc limit 10)
+union
+(select * from my_student where gender = 2 order by age asc limit 10);
++----+--------+----------+------+--------+
+| id | name   | class_id | age  | gender |
++----+--------+----------+------+--------+
+|  7 | 张飞   |        2 |   21 |      1 |
+|  2 | 李四   |        1 |   19 |      1 |
+|  1 | 刘备   |        1 |   18 |      2 |
+|  3 | 王五   |        2 |   20 |      2 |
+|  8 | 关羽   |        1 |   22 |      2 |
++----+--------+----------+------+--------+
+```
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