根据列枚举值创建SQL语句

Question

Let's assume I have a table called normalize with an enum column called type

Depending on the row (Let's assume the types are a, b or c) I want to do a LEFT JOIN based on whatever the type is.

So, if type = a, LEFT JOIN ON table a .. if type = B, LEFT JOIN ON table b... etc

Can this be done efficiently? Or should this be done at the application layer?

Answer 1

Yes, you should be able to do something like this:

SELECT *
FROM parent
    LEFT JOIN a ON (type = 'a' AND parent.id = a.id)
    LEFT JOIN b ON (type = 'b' AND parent.id = b.id)
    LEFT JOIN c ON (type = 'c' AND parent.id = c.id)

You may or may not need an index on type , depending on the actual data and how smart your DBMS is (in exploiting indexes that ought to exist underneath PKs). In any case, measure on realistic amounts of data.

Answer 2

One common approach is to store the type as a 1:1 relation. In this approach, there is no type enum column. The fact that an ID is present in a 1:1 extension table indicates its type.

select  *
from    BaseTable b
left join
        Type1Table t1
on      t1.Id = b.Id
left join
        Type1Table t2
on      t2.Id = b.Id

So an entity with an entry in b and t1 must be of type 1. An entity with an entry in b and in t2 is of type 2. That way, the information is only stored once, and an inconsistency where the table would say "type 1" but there was no matching row in t1 is impossible.

Answer 3

The problem is that even if you use SELECT table1.* FROM table1 LEFT OUTER JOIN table2 , the engine will evaluate the join even though you don't even need to show them.

So, in my opinion there are 2 options:

• 1st: Assume the joins are effective, using proper indexes and with an acceptable I/O, and use the following query:

** SEE BELOW, WITHIN THE LIST IT BUGS **

• Assume doing n joins, n being the number of different type (if there's a table per type), would give to much overhead, and then procede at the applicative layer with 2 queries: the first to get the type and then another within a switch case statement for example to retrieve datas from the right table.

Explain plan also could give you a good clue of what you're doing

Query:

SELECT
    IF(t.type = a, ta.value,
        IF(type = b, tb.value, IF(type = c, tc.value, td.value))
    )
FROM 
    table t
        LEFT JOIN type_a_table ta ON t.type = a AND ...
        LEFT JOIN type_a_table tb ON t.type = b AND ...
        LEFT JOIN type_a_table tc ON t.type = c AND ...
        LEFT JOIN type_a_table td ON t.type = d AND ...

Answer 4

Not sure if this helps, but I would probably use a union to perform what you're looking for

select * 
from BaseTable b left join TypeTableA a on b.id = a.id
union
select * 
from BaseTable b left join TypeTableB b on b.id = b.id