编写此查询的更好/更有效的方法

Question

I am trying to count the number of records in a table. The table is called affiliations and only has 4 columns (2 of which are foreign keys)

I want to count the number of records where the affiliated column is 0 and the business_id is related to a particular account_email.

I know how to do this query using the IN keyword, but I was wondering if there is a better or more efficient way to do this.

This is the IN version of the query:

SELECT COUNT(1) FROM affiliations 
WHERE business_id IN (
    SELECT business_id 
    FROM affiliations 
    WHERE account_email = 'address@domain.ext'
) AND affiliated = 0

I understand I could probably replace this with EXISTS:

SELECT COUNT(1) FROM affiliations 
WHERE EXISTS (
    SELECT 1 FROM affiliations 
    WHERE account_email = 'address@domain.ext'
) AND affiliated = 0

Would the statement with EXISTS work? And as previously asked, is there just a better way to do this?

Thanks in advance!

Answer 1

I'd use exists, but also remember to correlate the subquery to the primary table as below.

SELECT COUNT(1) FROM affiliations a
WHERE exists (
    SELECT 1
    FROM affiliations a1
    WHERE account_email = 'address@domain.ext'
    and a1.business_id=a.business_id
) AND affiliated = 0

Answer 2

The first query from the question with IN clause is not equivalent to the second with EXIST .

To convert the first query with IN , you must use a dependent subquery:

SELECT COUNT(1) FROM affiliations a1
WHERE EXISTS (
    SELECT 1 FROM affiliations a2
    WHERE account_email = 'address@domain.ext'
      AND a1.business_id = a2.business_id
) AND affiliated = 0

Pay attention to this condition: AND a1.business_id = a2.business_id

The above query is semantically eqivalent to your first query with IN .
Their performance are the same as well, because MySql, during the optimalization phase, converts intenrally a condition of this form:

outer_expr IN (SELECT inner_expr FROM ... WHERE subquery_where)

into this:

EXISTS (SELECT 1 FROM ... WHERE subquery_where AND outer_expr=inner_expr)

See this link for detalis: http://dev.mysql.com/doc/refman/5.0/en/subquery-optimization-with-exists.html
Pay speciall attention to discussion about NULL values and how NULL impacts the optimizer.

In short - if business_id column is declared as NOT NULL , then MySql is able to optimize these two queries.
See final conclusion (at the bottom of the page in this link):

To help the query optimizer better execute your queries, use these tips:

• A column must be declared as NOT NULL if it really is. (This also helps other aspects of the optimizer.)

• If you don't need to distinguish a NULL from FALSE subquery result, you can easily avoid the slow execution path. Replace a comparison that looks like this:

  outer_expr IN (SELECT inner_expr FROM ...)

  with this expression:

  (outer_expr IS NOT NULL) AND (outer_expr IN (SELECT inner_expr FROM ...))

  Then NULL IN (SELECT ...) will never be evaluated because MySQL stops evaluating AND parts as soon as the expression result is clear.

Answer 3

Use JOIN instead of IN. IN is terrible for performance if you're trying to match a lot of values

SELECT COUNT(1) 
FROM affiliations AS ABB2
    JOIN (SELECT business_id 
          FROM affiliations 
          WHERE account_email = 'address@domain.ext') AS ABB1
        ON ABB1.business_id = ABB2.business_id
WHERE affiliated = 0