Merge SQL Tables | 易学教程

问题

I have two tables, which have the same columns except for 1. So for example:

Table1 (column names): Student | Course | Exam1 | Exam2
Table2 (column names): Student | Course | Exam3 | FinalExam

I would like to combine these two tables to get:

Table: Student | Course | Exam1 | Exam2 | FinalExam

I have something along the following:

Select
    student,
    course,
    Exam1, 
    Exam2
From Table1

Select
    student,
    course,
    Exam3, 
    FinalExam
From Table2


Select
    student,
    course,
    Coalesce( t1.Exam1, 0) as Exam1 
    Coalesce( t1.Exam2, 0) as Exam2
    Coalesce( t2.Exam3, 0) as Exam3
    Coalesce( t2.FinalExam, 0) as FinalExam
From Table1 t1, Table2 t2

Is there a way to do this more efficiently/succinctly using an inner join?

回答1:

What you do is a cartesian product which will have n*n rows.

Try this

Select
    student,
    course,
    Exam1,
    Exam2,
    Exam3,
    FinalExam,
From Table1 t1, Table2 t2
Where t1.student=t2.student
and t1.course=t2.course;

This query works on the arrumption that the students have appeared for the either exams 1 or 2 atleast AND either exams 3 or Final. If there is a possible case of absent, then you need to use outer join. Like the following example, but not limited to

Select
    student,
    course,
    Exam1,
    Exam2,
    Exam3,
    FinalExam,
From Table1 t1, Table2 t2
Where t1.student=t2.student(+)
and t1.course=t2.course(+);

回答2:

Here is what I think you want based on some assumptions from your question.

 select isnull(t1.student, t2.student),  isnull(t1.course, t2.course),
    IsNull( t1.Exam1, 0) as Exam1 ,
    IsNull( t1.Exam2, 0) as Exam2,
    IsNull( t2.Exam3, 0) as Exam3,
IsNull( t2.FinalExam, 0) as FinalExam
From Table1 t1 
   full outer join  Table2 t2 
        on t1.student = t2.student 
            and t1.course = t2.course

来源：https://stackoverflow.com/questions/18836382/merge-sql-tables

标签

sql

inner-join