在MySQL 5.7中，我如何选择所有少于2个相关子表行的父表行

Question

Again this is a MySQL 5.7 DB. For example, let's say I have a database for a car lot, a table for cars and a table for price updates. (This is a contrived example).

Database: CarLot
Table: Cars
-Id
-Year
-Make
-Model

Table: PriceUpdates
-Id
-CarId (FK Cars:Id)
-Price
-DateChanged

To pseudo code what I want to do would look like:

SELECT * from Cars WHERE a Car has < 2 PriceUpdate rows (for each given CarId there are less than 2 associated PriceUpdate rows)

I'm not super strong in SQL and not sure if it's doable in a simple query or if I have to get into a procedure or T-SQL or something.

Answer 1

A join query with an assertion on the count should work here:

SELECT c.*
FROM Cars c
LEFT JOIN PriceUpdates pu
    ON c.Id = pu.CarId
GROUP BY
    c.Id
HAVING
    COUNT(pu.CarId) < 2;

Note: In MySQL (and ANSI SQL) it is completely valid to group by Cars.Id and select all other columms from this table because these columns are functionally dependent on the Id column.

Answer 2

If you wanted the prices as well (which seems likely despite your pseudo-code), then exists would often be used:

SELECT c.*, pu.*
FROM Cars c LEFT JOIN
     PriceUpdates pu
     ON c.Id = pu.CarId
WHERE NOT EXISTS (SELECT 1
                  FROM PriceUpdates pu2
                  WHERE pu2.CarId = pu.CarId AND
                        pu2.Id <> pu.Id
                 );