PostgreSQL将表联接到几列

Question

I have a list of students' name called table Names and I want to find their categories from another table called Categories as below:

Class_A    Class_B    Class_C  Class_D  Category
Sam                   Adam              High
            Sarah                       Medium
James                                   High
Emma        Simon               Nick    Low

My solution is to do a left join but students name from first table should be matching with one of four columns so I am not sure how to write queries. At the moment my query is just matching to Class_A while I need to check all categories and if the student's name exist, return category. (Note: some rows have more than one student's name)

SELECT Names.name, Categories.Category 
FROM Names
   LEFT JOIN Categories ON Names.name = Categories.Class_A; 

Table Names looks like this:

Name
----
Emma
Nick
James
Adam
Jack
Sarah

And I am expecting an output as below:

Name     Category
----     ----
Emma     Low
Nick     Low
James    High
Adam     High
Jack     -
Sarah    Medium

Answer 1

Try this using OR in on clause:

SELECT Names.name, coalesce(Categories.Category,'-') as category 
FROM Names
   LEFT JOIN Categories ON Names.name = Categories.Class_A or Names.name = Categories.Class_B or Names.name = Categories.Class_C or Names.name = Categories.Class_D

Answer 2

I would be inclined to unpivot the first table. This looks like:

select n.name, c.category
from name n left join
     (categories c cross join lateral
      (values (c.class_a), (c.class_b), (c.class_c), (c.class_d)
      ) v(name)
     )
     on n.name = v.name
where v.name is not null;

Although you can also solve this using in (or or ) in the on clause, that may produce a much less efficient execution plan.