SQL如何查找多个表中未使用的键

Question

SQL, a table (thesau) with terms that may or may not be in use in several other tables. The other tables point to the id in thesau. I want to find out which terms are NOT in use, ie which terms are not referred to by any other table. Simplified:

table: thesau
id term
1  painting
2  sari
4  oil
5  silk
8  gouache

table: object_type
id
1   (-> painting)
7   (-> ... )

table: material_type:
id
2   (-> silk)
4   (-> oil)

in collection, object_type 1 refers to thesau 1 = painting, and so on. Now, I can find thesau terms not in use in one single table, like so:

select distinct thesau.id, thesau.term from thesau_term 
  where thesau_term.id not in 
  (select object_type.id from object_type)

This works. I want to expand the same query to other tables, in one query if possible. In pseudo-code:

select distinct id, term from thesau_term 
  where thesau_term.id not in 
  ((select object_type.id from object_type) or
   (select material_type.id from material_type))

This doesn't work. What am I missing?

Answer 1

Your current query would list a term that is not in use in at least one other table. If you use AND instead of OR , your query would list terms not in use in any other table:

select distinct id, term from thesau_term 
  where thesau_term.id not in 
  ((select object_type.id from object_type) AND -- <<== HERE
   (select material_type.id from material_type))

You can further simplify your query by using NOT EXISTS

SELECT id, term
FROM thesau_term t
WHERE
    NOT EXISTS (SELECT * FROM object_type ot WHERE ot.id=t.id)
AND NOT EXISTS (SELECT * FROM material_type mt WHERE mt.id=t.id)

or by using an outer join

SELECT DISTINCT t.id, t.term
FROM thesau_term t
LEFT OUTER JOIN object_type ot ON ot.id=t.id
LEFT OUTER JOIN material_type mt ON mt.id=t.id
WHERE ot.id IS NULL and mt.id IS NULL

Answer 2

You don't need to use a join , you can do what you have done in your first query, you just need to change the where clause. A not exists would be the most permanent but something like this would work:

SELECT DISTINCT
       id,
       term
FROM thesau_term
WHERE thesau_term.id NOT IN
(
    SELECT object_type.id
    FROM object_type
)
      OR thesau_term.id NOT IN
(
    SELECT material_type.id
    FROM material_type
);