【连接查询】mySql多表连接查询与union与union all用法

1.准备两个表

表a:

　　结构:

mysql> desc a;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| name  | varchar(40) | NO   | PRI | NULL    |       |
| age   | int(11)     | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

　　数据

表b:

　　结构

mysql> desc b;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| nameB | varchar(40) | YES  |     | NULL    |       |
| ageB  | int(11)     | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

　　数据:

2.进行连接查询测试:

(1)交叉连接(笛卡尔积)  cross join

mysql> select * from a,b;　　#第一种
+------+------+-------+------+
| name | age  | nameB | ageB |
+------+------+-------+------+
| A1   |    1 | B1    |    1 |
| A2   |    2 | B1    |    1 |
| A1   |    1 | B2    |   22 |
| A2   |    2 | B2    |   22 |
+------+------+-------+------+
4 rows in set (0.00 sec)

mysql> select * from a cross join b;　　#第二种
+------+------+-------+------+
| name | age  | nameB | ageB |
+------+------+-------+------+
| A1   |    1 | B1    |    1 |
| A2   |    2 | B1    |    1 |
| A1   |    1 | B2    |   22 |
| A2   |    2 | B2    |   22 |
+------+------+-------+------+
4 rows in set (0.00 sec)

mysql> select a.*,b.* from a cross join b;　　#第二种的又一个写法
+------+------+-------+------+
| name | age  | nameB | ageB |
+------+------+-------+------+
| A1   |    1 | B1    |    1 |
| A2   |    2 | B1    |    1 |
| A1   |    1 | B2    |   22 |
| A2   |    2 | B2    |   22 |
+------+------+-------+------+
4 rows in set (0.00 sec)

 (2)内连接    join 或 inner join(在笛卡尔积的基础上过滤)

• 显示内连接

(1)不带条件的内连接

mysql> select a.*,b.* from a inner join b on a.age=b.ageb;　　#第一种 inner join
+------+------+-------+------+
| name | age  | nameB | ageB |
+------+------+-------+------+
| A1   |    1 | B1    |    1 |
+------+------+-------+------+
1 row in set (0.00 sec)

mysql> select a.*,b.* from a join b on a.age=b.ageb;　　#第二种  join (默认是inner join)
+------+------+-------+------+
| name | age  | nameB | ageB |
+------+------+-------+------+
| A1   |    1 | B1    |    1 |
+------+------+-------+------+
1 row in set (0.00 sec)

三个表的显示内连接:

SELECT
  a.*,
  b.*,
  c.*
FROM exampaper a
  INNER JOIN bigquestion b
  INNER JOIN exampaperquestion c
    ON a.paperId = b.paperId
      AND b.bigQuertionId = c.bigQuertionId

四个表的显示内连接:

SELECT
    train.trainingSchemaName,
    train.majorName,
    train.createTime,
    tc.*, course.*, type.*
FROM
    trainschemeinfo train
JOIN train_course tc ON train.trainingSchemeID = tc.trainningSchemeID
INNER JOIN t_course_base_info course ON tc.courseID = course.courseId
INNER JOIN coursetypeinfo type ON tc.typeNum = type.typeNum
WHERE
    tc.trainningSchemeID = '661ecb064b164d1ea133956f89beddb7'

与之等价的隐士内连接:

SELECT
    train.trainingSchemaName,
    train.majorName,
    train.createTime,
    tc.*, course.*, type.*
FROM
    trainschemeinfo train,
    train_course tc,
    t_course_base_info course,
    coursetypeinfo type
WHERE
    train.trainingSchemeID = tc.trainningSchemeID
AND tc.courseID = course.courseId
AND tc.typeNum = type.typeNum
AND tc.trainningSchemeID = '661ecb064b164d1ea133956f89beddb7'

(2)显示内连接带条件

mysql> select a.*,b.* from a join b on a.age=b.ageb having a.name='A1';　　#having从查出的数据中挑选满足条件的元祖
+------+------+-------+------+
| name | age  | nameB | ageB |
+------+------+-------+------+
| A1   |    1 | B1    |    1 |
+------+------+-------+------+
1 row in set (0.00 sec)
　　
mysql> select a.*,b.* from a join b on a.age=b.ageb where a.name='A1';　　#where查询满足条件的元素
+------+------+-------+------+
| name | age  | nameB | ageB |
+------+------+-------+------+
| A1   |    1 | B1    |    1 |
+------+------+-------+------+
1 row in set (0.00 sec)

• 隐士内连接:

mysql> select * from a,b where a.age=b.ageb;　　
+------+------+-------+------+
| name | age  | nameB | ageB |
+------+------+-------+------+
| A1   |    1 | B1    |    1 |
+------+------+-------+------+
1 row in set (0.00 sec)

mysql> select * from a,b where a.age=b.ageb and a.name='A1';
+------+------+-------+------+
| name | age  | nameB | ageB |
+------+------+-------+------+
| A1   |    1 | B1    |    1 |
+------+------+-------+------+
1 row in set (0.00 sec)

mysql> select * from a,b where a.age=b.ageb having a.name='A1';
+------+------+-------+------+
| name | age  | nameB | ageB |
+------+------+-------+------+
| A1   |    1 | B1    |    1 |
+------+------+-------+------+
1 row in set (0.00 sec)

　　where是从本地磁盘查询满足条件的元素，having是从查出的数据中挑选满足条件的元素。执行权限   where>sum()..聚合函数>having

　　

 (3)左外连接:(拿左边的匹配右边的，没有找到右边的为null)

mysql> select * from a left join b on a.age = b.ageb;　　#第一种 left join
+------+------+-------+------+
| name | age  | nameB | ageB |
+------+------+-------+------+
| A1   |    1 | B1    |    1 |
| A2   |    2 | NULL  | NULL |
+------+------+-------+------+
2 rows in set (0.00 sec)

mysql> select * from a left outer join b on a.age = b.ageb;　　#第二种 left outer join
+------+------+-------+------+
| name | age  | nameB | ageB |
+------+------+-------+------+
| A1   |    1 | B1    |    1 |
| A2   |    2 | NULL  | NULL |
+------+------+-------+------+
2 rows in set (0.00 sec)

(4)右外连接:(拿右边的匹配左边的，没有找到左边的为null)

mysql> select * from a right join b on a.age = b.ageb;　　#第一种  right join
+------+------+-------+------+
| name | age  | nameB | ageB |
+------+------+-------+------+
| A1   |    1 | B1    |    1 |
| NULL | NULL | B2    |   22 |
+------+------+-------+------+
2 rows in set (0.00 sec)

mysql> select * from a right outer join b on a.age = b.ageb;　　#第二种 right outer join
+------+------+-------+------+
| name | age  | nameB | ageB |
+------+------+-------+------+
| A1   |    1 | B1    |    1 |
| NULL | NULL | B2    |   22 |
+------+------+-------+------+
2 rows in set (0.00 sec)

3.Union 和 union all  取两个表的并集测试

修改b表，加一条和a表重复的数据

b表数据:

a表数据:

(1)   union:   自动去掉重复元素

mysql> select * from a union select * from b;
+------+------+
| name | age  |
+------+------+
| A1   |    1 |
| A2   |    2 |
| B1   |    1 |
| B2   |   22 |
+------+------+
4 rows in set (0.00 sec)

 总结:

union:联合的意思，即把两次或多次查询结果合并起来。
要求：两次查询的列数必须一致
推荐：列的类型可以不一样，但推荐查询的每一列，想对应的类型以一样
可以来自多张表的数据：多次sql语句取出的列名可以不一致，此时以第一个sql语句的列名为准。
如果不同的语句中取出的行，有完全相同(这里表示的是每个列的值都相同)，那么union会将相同的行合并，最终只保留一行。也可以这样理解，union会去掉重复的行。
如果不想去掉重复的行，可以使用union all。
如果子句中有order by,limit，需用括号()包起来。推荐放到所有子句之后，即对最终合并的结果来排序或筛选，可以对union之后的数据进行排序和分页等操作。

例如:采用union合并的多个表的数据的SQL

　　需求是:为了显示学院、专业、班级树，但是这些信息不在一个月表，而且班级表中有专业编号，专业表中有学院编号。思路就是:分别从三个表中获取数据，然后采用union进行合并数据。

SELECT
  classID   AS departNum,
  className AS departName,
  "class" AS departType,
  (SELECT
     majorID
   FROM t_major_base_info
   WHERE majorID = class.majorID) AS updepartNum
FROM t_class_base_info class
UNION
SELECT majorID AS departNum,
    majorName AS departName,
    "major" AS departType,
      (SELECT
        collegeID   FROM t_college_base_info
   WHERE collegeID=major.collegeID) AS updepartNum
 FROM t_major_base_info major
UNION
SELECT collegeId AS departNum,
    collegeName AS departName,
    "college" AS departType,
    "000" AS updepartNum
    FROM t_college_base_info

(2)    union all　　保留重复元素

 　　UNION ALL 命令和 UNION 命令几乎是等效的，不过 UNION ALL 命令会列出所有的值。

mysql> select * from a union all select * from b;
+------+------+
| name | age  |
+------+------+
| A1   |    1 |
| A2   |    2 |
| B1   |    1 |
| B2   |   22 |
| A1   |    1 |
+------+------+
5 rows in set (0.00 sec)

　　

总结:

　　UNION 用于合并两个或多个 SELECT 语句的结果集，并消去表中任何重复行。
　　UNION 内部的 SELECT 语句必须拥有相同数量的列，列也必须拥有相似的数据类型。
　　同时，每条 SELECT 语句中的列的顺序必须相同.

　　默认地，UNION 操作符选取不同的值。如果允许重复的值，请使用 UNION ALL。
　　当 ALL 随 UNION 一起使用时（即 UNION ALL），不消除重复行

注意：

1、UNION 结果集中的列名总是等于第一个 SELECT 语句中的列名
2、UNION 内部的 SELECT 语句必须拥有相同数量的列。列也必须拥有相似的数据类型。同时，每条 SELECT 语句中的列的顺序必须相同

补充:今天在项目种用到了从多个表种查询数据把并且分页，实现过程是将所有表的名字传到dao，然后遍历表名字，一起添加条件(前提是需要查询的列名字相同)：

    public List<Map<String, Object>> getAllData(List<String> tableNames) {
        List<String> sqls = new ArrayList();
        StringBuilder sb = null;
        for (String tableName : tableNames) {
            sb = new StringBuilder();
            sb.append("select id,name from ");
            sb.append(tableName);
            sb.append(" where 1=1");
            sb.append(" and name = 'zhangsan'");
            sqls.add(sb.toString());
        }

        String sqlFinally = StringUtils.join(sqls, " union ");
        sqlFinally += "order by name limit 5,5";
        System.out.println(sqlFinally);

        /*Session session = getSessionFactory().openSession();
        SQLQuery sqlQuery = session.createSQLQuery(sqlFinally);
        sqlQuery.setResultTransformer(Transformers.ALIAS_TO_ENTITY_MAP);
        return sqlQuery.list();*/
        return null;

    }

测试：

    public static void main(String[] args) {
        GroupDaoImpl g = new GroupDaoImpl();
        List tableNames = new ArrayList();
        tableNames.add("t1");
        tableNames.add("t2");
        tableNames.add("t3");
        g.getAllData(tableNames);
    }

结果:

select id,name from t1 where 1=1 and name = 'zhangsan' union select id,name from t2 where 1=1 and name = 'zhangsan' union select id,name from t3 where 1=1 and name = 'zhangsan' order by name limit 5,5