Question

我不确定我想要使用联合做什么，或者我是否需要使用嵌套查询和某种联接。

select c1,c2 from t1
union
select c1,c2 from t2
// with some sort of condition where t1.c1 = t2.c1

示例：

t1
| 100 | regular |
| 200 | regular |
| 300 | regular |
| 400 | regular |

t2
| 100 | summer |
| 200 | summer |
| 500 | summer |
| 600 | summer |

Desired Result
| 100 | regular |
| 100 | summer  |
| 200 | regular |
| 200 | summer  |

我尝试过类似的事情：

select * from (select * from t1) as q1
inner join
    (select * from t2) as q2 on q1.c1 = q2.c1

但是这会将记录加入到这样的单行中：

| 100 | regular | 100 | summer |
| 200 | regular | 200 | summer |

Answer 1

尝试：

select c1, c2
  from t1
 where c1 in (select c1 from t2)
union all
select c1, c2
  from t2
 where c1 in (select c1 from t1)

根据编辑，请尝试以下操作：

MySQL没有WITH子句允许你多次引用你的t1和t2 subs。您可能希望在数据库中创建t1和t2作为视图，以便在单个查询中多次将它们称为t1和t2。

即便如此，下面的查询老实地看起来非常糟糕，如果我们知道您的数据库结构，可能会进行很多优化。 IE浏览器。表的列表，每个表上的所有列及其数据类型，每个表的几个示例行以及您的预期结果。

例如在你的t1子中你有一个带有LESSON表的外连接，但是你的WHERE子句中有标准（lesson.dayofweek＆gt; = 0），这自然不允许空值，有效地转动你的外表加入内部联接。此外，您还有子查询仅使用标准来检查studentid是否存在，这些标准表明实际上不需要使用多个表来生成所需的结果。但是，如果不知道您的数据库结构和一些具有预期结果的示例数据，则很难进一步建议。

即使如此，我相信下面的内容可能会让你得到你想要的东西，而不是最佳。

select *
  from (select distinct students.student_number as "StudentID",
                        concat(students.first_name, ' ', students.last_name) as "Student",

                        general_program_types.general_program_name as "Program Category",
                        program_inventory.program_code as "Program Code",
                        std_lesson.studio_name as "Studio",
                        concat(teachers.first_name, ' ', teachers.last_name) as "Teacher",

          from lesson_student
          left join lesson
            on lesson_student.lesson_id = lesson.lesson_id
          left join lesson_summer
            on lesson_student.lesson_id = lesson_summer.lesson_id

         inner join students
            on lesson_student.student_number = students.student_number
         inner join studio as std_primary
            on students.primary_location_id = std_primary.studio_id
         inner join studio as std_lesson
            on (lesson.studio_id = std_lesson.studio_id or
               lesson_summer.studio_id = std_lesson.studio_id)

         inner join teachers
            on (lesson.teacher_id = teachers.teacher_id or
               lesson_summer.teacher_id = teachers.teacher_id)
         inner join lesson_program
            on lesson_student.lesson_id = lesson_program.lesson_id
         inner join program_inventory
            on lesson_program.program_code_id =
               program_inventory.program_code_id
         inner join general_program_types
            on program_inventory.general_program_id =
               general_program_types.general_program_id

         inner join accounts
            on students.ACCOUNT_NUMBER = accounts.ACCOUNT_NUMBER
         inner join account_contacts
            on students.ACCOUNT_NUMBER = account_contacts.ACCOUNT_NUMBER

        /** NOTE: the WHERE condition is the only **/
        /** difference between subquery1 & subquery2 **/
         where lesson.dayofweek >= 0 and
         order by students.STUDENT_NUMBER) t1
 where StudentID in
       (select StudentID
          from (select distinct students.student_number as "StudentID",
                                concat(students.first_name,
                                       ' ',
                                       students.last_name) as "Student",

                                general_program_types.general_program_name as "Program Category",
                                program_inventory.program_code as "Program Code",
                                std_lesson.studio_name as "Studio",
                                concat(teachers.first_name,
                                       ' ',
                                       teachers.last_name) as "Teacher",

                  from lesson_student
                  left join lesson
                    on lesson_student.lesson_id = lesson.lesson_id
                  left join lesson_summer
                    on lesson_student.lesson_id = lesson_summer.lesson_id

                 inner join students
                    on lesson_student.student_number =
                       students.student_number
                 inner join studio as std_primary
                    on students.primary_location_id = std_primary.studio_id
                 inner join studio as std_lesson
                    on (lesson.studio_id = std_lesson.studio_id or
                       lesson_summer.studio_id = std_lesson.studio_id)

                 inner join teachers
                    on (lesson.teacher_id = teachers.teacher_id or
                       lesson_summer.teacher_id = teachers.teacher_id)
                 inner join lesson_program
                    on lesson_student.lesson_id = lesson_program.lesson_id
                 inner join program_inventory
                    on lesson_program.program_code_id =
                       program_inventory.program_code_id
                 inner join general_program_types
                    on program_inventory.general_program_id =
                       general_program_types.general_program_id

                 inner join accounts
                    on students.ACCOUNT_NUMBER = accounts.ACCOUNT_NUMBER
                 inner join account_contacts
                    on students.ACCOUNT_NUMBER =
                       account_contacts.ACCOUNT_NUMBER

                /** NOTE: the WHERE condition is the only **/
                /** difference between subquery1 & subquery2 **/
                 where lesson_summer.dayofweek >= 0
                 order by students.STUDENT_NUMBER) t2)
UNION ALL
select *
  from (select distinct students.student_number as "StudentID",
                        concat(students.first_name, ' ', students.last_name) as "Student",

                        general_program_types.general_program_name as "Program Category",
                        program_inventory.program_code as "Program Code",
                        std_lesson.studio_name as "Studio",
                        concat(teachers.first_name, ' ', teachers.last_name) as "Teacher",

          from lesson_student
          left join lesson
            on lesson_student.lesson_id = lesson.lesson_id
          left join lesson_summer
            on lesson_student.lesson_id = lesson_summer.lesson_id

         inner join students
            on lesson_student.student_number = students.student_number
         inner join studio as std_primary
            on students.primary_location_id = std_primary.studio_id
         inner join studio as std_lesson
            on (lesson.studio_id = std_lesson.studio_id or
               lesson_summer.studio_id = std_lesson.studio_id)

         inner join teachers
            on (lesson.teacher_id = teachers.teacher_id or
               lesson_summer.teacher_id = teachers.teacher_id)
         inner join lesson_program
            on lesson_student.lesson_id = lesson_program.lesson_id
         inner join program_inventory
            on lesson_program.program_code_id =
               program_inventory.program_code_id
         inner join general_program_types
            on program_inventory.general_program_id =
               general_program_types.general_program_id

         inner join accounts
            on students.ACCOUNT_NUMBER = accounts.ACCOUNT_NUMBER
         inner join account_contacts
            on students.ACCOUNT_NUMBER = account_contacts.ACCOUNT_NUMBER

        /** NOTE: the WHERE condition is the only **/
        /** difference between subquery1 & subquery2 **/
         where lesson_summer.dayofweek >= 0
         order by students.STUDENT_NUMBER) x
 where StudentID in
       (select StudentID
          from (select distinct students.student_number as "StudentID",
                                concat(students.first_name,
                                       ' ',
                                       students.last_name) as "Student",

                                general_program_types.general_program_name as "Program Category",
                                program_inventory.program_code as "Program Code",
                                std_lesson.studio_name as "Studio",
                                concat(teachers.first_name,
                                       ' ',
                                       teachers.last_name) as "Teacher",

                  from lesson_student
                  left join lesson
                    on lesson_student.lesson_id = lesson.lesson_id
                  left join lesson_summer
                    on lesson_student.lesson_id = lesson_summer.lesson_id

                 inner join students
                    on lesson_student.student_number =
                       students.student_number
                 inner join studio as std_primary
                    on students.primary_location_id = std_primary.studio_id
                 inner join studio as std_lesson
                    on (lesson.studio_id = std_lesson.studio_id or
                       lesson_summer.studio_id = std_lesson.studio_id)

                 inner join teachers
                    on (lesson.teacher_id = teachers.teacher_id or
                       lesson_summer.teacher_id = teachers.teacher_id)
                 inner join lesson_program
                    on lesson_student.lesson_id = lesson_program.lesson_id
                 inner join program_inventory
                    on lesson_program.program_code_id =
                       program_inventory.program_code_id
                 inner join general_program_types
                    on program_inventory.general_program_id =
                       general_program_types.general_program_id

                 inner join accounts
                    on students.ACCOUNT_NUMBER = accounts.ACCOUNT_NUMBER
                 inner join account_contacts
                    on students.ACCOUNT_NUMBER =
                       account_contacts.ACCOUNT_NUMBER

                /** NOTE: the WHERE condition is the only **/
                /** difference between subquery1 & subquery2 **/
                 where lesson.dayofweek >= 0 and
                 order by students.STUDENT_NUMBER) x);

Answer 2

为了比较两个表中的值，您需要进行连接。我看到的第一种方法是使用内部联接进行2次查询并将它们联合起来：

select t1.* from t1
inner join t2 using (c1)
union
select t2.* from t1
inner join t2 using (c1)

但是，我们可以做得更好：您可以看到我使用完全相同的查询，直到我在结果集中选择哪些列，这有点浪费。如果我们可以简单地复制每个记录^（1）？好吧，我们可以：

select * from t1
inner join t2 using (c1)
cross join (
    select 0 as parity
    union
    select 1 as parity) dup

现在我们将所有内容都设置为double，并且有一个额外的奇偶校验列，我们只需要在其上选择一组列：

-- broken query
select case parity when 
   0 then t1.*
   1 then t2.*
end from t1
inner join t2 using (c1)
cross join (
    select 0 as parity
    union
    select 1 as parity) dup

不幸的是，case表达式不允许返回多个列，因此我们需要为所有列包含测试。在您的情况下，一列足够了：

select c1, case parity when 
   0 then t1.c2
   1 then t2.c2
end from t1
inner join t2 using (c1)
cross join (
    select 0 as parity
    union
    select 1 as parity) dup
order by c1
-- order by parity, c1

使用已注释的order by来获取t1的结果集，然后是t2。

警告：代码未经测试。

^{（1）这个在ANSI SQL中称为CTE的功能（99我认为）允许分解出一个公共查询，然后在整个主查询中引用它，但遗憾的是mysql不支持它。}

Answer 3

UNION没问题。

您希望运行相同的加入两次。除了第一次超过你，左边第二次。

SELECT t1.* FROM t1 JOIN t2 USING (c1)
UNION
SELECT t2.* FROM t1 JOIN t2 USING (c1)

当然，如果可能的话，您可以运行单个查询并将左侧保存在内存中，显示右侧，然后将保存的左侧排队。它需要比游标更多的内存，但是在一半的时间内运行查询（实际上更少，因为磁盘和资源缓存）。

请参阅here示例SQLFiddle。

在MySQL中为UNION添加条件

3 个答案: