Question

I have a table structure like the following:

table a:
id int
name varchar

table b:
id int
name varchar

table c:
id int
valid tinyint
table_a int
table_b int

Basically, table c maps between table a and b and also has a column to determine if something is valid (either a 1 or 0). I want to be able to select every name (only once) from table a and the linked ‘valid’ column from table c (and default to 0 if it doesn’t exist) and limit it by matching table_b to an id. Hopefully that makes sense.

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Here is some example data to help show what I mean:

Using the table structure as above I have this data:

Table a:
(id, name)
1, row 1
2, row 2
3, row 3

Table b:
(id, name)
1, row a
2, row b

Table c:
(id, valid, table_a, table_b)
1, 1, 1, 1

Now, what I want to select is all of the names from table a, the ‘valid’ column from c (default to 0 if no row that matches) and limit the ‘valid’ fields by only those that table_b = 1. Hopefully that makes more sense?

So, I want back the following where b.id = 1:

(a.name, c.valid)
row 1, 1
row 2, 0
row 3, 0

Answer 1

Let me see if I got it. Are you looking for something like this?

This query:

select a.name, coalesce(c.valid, 0) as IsValid, b.id from a
left join c on a.id = c.table_a
left join b on b.id = c.table_b

Results in this:

NAME    ISVALID ID
row 1   1       1
row 2   0   
row 3   0   

If you want all b.id = 1 then you WILL NOT have ALL rows from table_a.

select a.name, coalesce(c.valid, 0) as IsValid from a
left join c on a.id = c.table_a
left join b on b.id = c.table_b
where b.id = 1

NAME    ISVALID ID
row 1   1       1