How can I make a WHERE clause only apply to the right table in a left join?

问题

I have two tables.

TableA: field_definitions
field_id, field_type, field_length, field_name, field_desc, display_order, field_section, active

TableB: user_data
response_id, user_id, field_id, user_response

I need a query that will return all rows from table A and, if they exist, matching rows from table B based on a particular user_id.

Here is what I have so far...

SELECT field_definitions. * , user_data.user_response
FROM field_definitions
LEFT JOIN user_data
USING ( field_id ) 
WHERE (
user_data.user_id =8
OR user_data.user_id IS NULL
)
AND field_definitions.field_section =1
AND field_definitions.active =1
ORDER BY display_order ASC

This only works if table B has zero rows or matching rows for the user_id in the WHERE clause. If table B has rows with matching field_id but not user_id, I get zero returned rows.

Essentially, once rows in table B exist for user X, the query no longer returns rows from table A when searching for user Z responses and none are found.

I need the result to always contain rows from table A even if there are no matching rows in B with the correct user_id.

回答1:

You can move those constraints from the WHERE clause to the ON clause (which first requires that you change the USING clause into an ON clause: ON clauses are much more flexible than USING clauses). So:

SELECT field_definitions.*,
       user_data.user_response
  FROM field_definitions
  LEFT
  JOIN user_data
    ON user_data.field_id = field_definitions.field_id
   AND user_data.user_id = 8
 WHERE field_definitions.field_section = 1
   AND field_definitions.active = 1
 ORDER
    BY field_definitions.display_order ASC
;

回答2:

Conceptually, the join is performed first and then the where clause is applied to the virtual resultset. If you want to filter one table first, you have to code that as a sub-select inside the join. Something along these lines:

SELECT 
  field_definitions. * , 
  user8.user_response
FROM 
  field_definitions
    LEFT JOIN (select * from user_data where user_id=8 or user_id is null) as user8
    USING ( field_id ) 
WHERE
  field_definitions.field_section =1
  AND field_definitions.active =1
ORDER BY display_order ASC

回答3:

You can move the WHERE clause inside as follows

SELECT field_definitions. * , user_data.user_response
FROM (
  select * from
  field_definitions
  WHERE field_definitions.field_section =1
  AND field_definitions.active =1 ) as field_definitions
LEFT JOIN (
  select * from
  user_data
  where user_data.user_id =8
  OR user_data.user_id IS NULL ) as user_data
USING ( field_id ) 
ORDER BY display_order ASC

回答4:

A literal translation of the sepc:

SELECT field_definitions. * , '{{MISSING}}' AS user_response
  FROM field_definitions 
UNION
SELECT field_definitions. * , user_data.user_response
  FROM field_definitions 
       NATURAL JOIN user_data
 WHERE user_data.user_id = 8;

However, I suspect that you don't really want "all rows from table A".

来源：https://stackoverflow.com/questions/9883156/how-can-i-make-a-where-clause-only-apply-to-the-right-table-in-a-left-join

标签

mysql

sql

join

left-join