MySQL：在一个查询中从四个不同表中选择结果

Question

I have four different tables, one main SPECIAL table containing only id's that reference to the CONTRACT and PHONE tables.

My query looks as follows:

SELECT *
FROM `specials` specials
INNER JOIN `contract` contracts
ON  specials.contract_id = contracts.id
INNER JOIN `phone` phones
ON specials.phone_id = phones.id
INNER JOIN `ugets` ugets
ON specials.id = ugets.special_id

At the moment, this only gets ONE row from the UGETS table, but I need all rows from this specific table with the correct special_id .

Can anyone please point me in the correct direction? I can't seem to help myself with Googling it.

Answer 1

INNER JOIN only returns rows where at least one match in both joined tables is available. The problem is that you are joining 4 tables together. So there could be more records (than 1) if you join only specials with ugets but these records get filtered out if you join with your other tables.

If it is not necessary that there is an existing record for every specials-entry in phones or contracts then change the joining type for these 2 tables to left join. (as unspecific hinted by comment of Dr. biziclop)

In general you should consider which of your joined data is optional and which is required. Your example query requires all data of your tables. Maybe this is not what you intended.

Answer 2

Your query looks about right, unless you also want all rows, regardless of the existence of a contract or phone . In this case, you'll want to use OUTER JOIN or LEFT JOIN instead of INNER JOIN :

SELECT *
FROM `specials` specials
LEFT JOIN `contract` contracts
ON  specials.contract_id = contracts.id
LEFT JOIN `phone` phones
ON specials.phone_id = phones.id
LEFT JOIN `ugets` ugets
ON specials.id = ugets.special_id

Fields from tables where there are not records for a given special will be returned with NULL , but all rows from tables where there's a match will be shown.

Beware this could have adverse performance impacts (possibly to the extent it voids the benefits of getting all four tables in a single query) - you should evaluate the gains vs. costs and see if you still want a single query.

Answer 3

As an alternative to LEFT JOINing, you could consider using UNION ALL to join the results of three queries, each inner joining the specials table to one of the other three tables.

This will ensure that you only return n(1)+n(2)+n(3) rows for each row on the specials table, instead of n(1)*n(2)*n(3) rows.