SQL查询提取具有相同WHERE条件的硬编码记录和金额

Question

I have a car database and I am trying to pull one of each make and model of cars. Example: There are three Toyota Corollas, four Honda Civics and five Acura RSX. The query should return one Toyota Corolla, one Honda Civic and one Acura RSX. I can code this however there is a lot of repetition of code in the query.

    SELECT
        *
    FROM
        main
    WHERE
        ( make_name = 'TOYOTA' AND model_name = 'CELICA' AND chassis = 'GT4' )
        AND
        mileage <= 100000
        AND
        rate >= '3.5'
        AND
        ( year >= 1990 and year <= 1998 )
    ORDER BY
        auction_date DESC,
        rate DESC
    LIMIT
        1
)
UNION
(
    SELECT
        *
    FROM
        main
    WHERE
        ( make_name = 'NISSAN' AND model_name = 'SKYLINE' AND chassis = 'R33' )
        AND
        mileage <= 100000
        AND
        rate >= '3.5'
        AND
        ( year >= 1990 and year <= 1998 )
    ORDER BY
        auction_date DESC,
        rate DESC
    LIMIT
        1
)

So this will pull one Toyota Celica GT4 and one Nissan Skyline R33. But I repeat the conditions of mileage, rate and year for each car. This query has two cars I need but essentially I will be adding more in over time so I would like a way to add to this query without repeating the WHERE condition for each car since they will always be the same.

Answer 1

SQL Statement

select 
*
from
(
SELECT make_name,
model_name,
chassis,
max(concat(auction_date,rate)) as mc
from main 
where
mileage <= 100000
        AND
        rate >= '3.5'
        AND
        ( year >= 1990 and year <= 1998 )  
group by
make_name, model_name, chassis 

) as a,
main as m
where a.make_name =m.make_name AND
a.model_name = m.model_name AND
a.chassis = m.chassis AND
a.mc = concat(m.auction_date,m.rate);

Explanation

This uses a derived table containing the distinct combinations of make_name , model_name , and chassis , as well as the MAXIMUM auction_date+rate .

I use the MAXIMUM auction_date+rate since we have the requirement is to get a only one ROW for each make_name, model_name, and chassis, but to get that row that has the LATEST auction_date, and highest rate.

This produces the derived table with results:

MAKE_NAME      MODEL_NAME   CHASSIS         MC
NISSAN         SKYLINE       GT4        0005-06-0117
NISSAN         SKYLINE       R33        0002-04-011
TOYOTA         CELICA        GT4        0005-01-0117

I then join this derived table with the main table, but with the conditions:

where a.make_name =m.make_name AND
a.model_name = m.model_name AND
a.chassis = m.chassis AND
a.mc = concat(m.auction_date,m.rate);

In this way, the rows are extracted that have the make, model, and chassis with the latest aucion date, and highest rate (But only one row for each make, model, and chassis).

SQLFiddle: http://sqlfiddle.com/#!2/1b5c8/2

Update

I want to specify in the SQL which unique models I want. Which was why I went: WHERE model_name='GT4' etc..

You can do this using multiple OR statements or using CONCAT and IN (or simply IN ).

So, you either append at the end:

 AND 
 (
  (m.model_name='CELICA' AND m.chassis='GT4') OR
  (m.model_name='TEST' AND m.chassis='TEST')
);

or you append:

AND concat(m.model_name,'-',m.chassis) in ('CELICA-GT4','SKYLINE-GT4');

See: http://sqlfiddle.com/#!2/6fc09/2 that has both examples.

Answer 2

Because you are using a LIMIT clause to restrict the number of rows returned, and because the LIMIT clause gets evaluated last (or very nearly last) in the execution plan, the WHERE clause gets evaluated before the LIMIT , so that WHERE clause has to be repeated in each query.

There are other approaches to returning an equivalent resultset which would avoid repeating some of the conditions in the where clause, but they won't be as efficient, and they won't be able to make use of the ORDER BY ... LIMIT ... construct to restrict the rows returned.