Get multiple columns from a select subquery

  • SELECT 
       *, 
       p.name AS name, 
       p.image, 
       p.price,
       ( 
           SELECT ps.price 
           FROM product_special ps 
           WHERE p.id = ps.id
             AND ps.date < NOW() 
           ORDER BY ps.priority ASC, LIMIT 1
       ) AS special_price,
       ( 
           SELECT ps.date 
           FROM product_special ps 
           WHERE p.id = ps.id
             AND ps.date < NOW() 
           ORDER BY ps.priority ASC, LIMIT 1
       ) AS date
    FROM product p LEFT JOIN product_special ps ON (p.id = ps.id)
    

    As you can see I'm repeating the same subquery just to get another column out. I'm wondering is there a better way of doing this?

    id is the primary key in both tables. I've no problem making product_special.priority unique if that can help.

  • Assuming combination product_special.id, product_special.priority is unique

     SELECT p.*, special_price,special_date
     FROM product p
     LEFT JOIN 
     (
         SELECT ps.id, ps.price as special_price, ps.`date` as special_date
         FROM product_special ps
         INNER JOIN 
         (
           SELECT id, MIN(priority) as min_priority 
           FROM product_special
           GROUP BY id
         ) ps2 
         ON (ps2.id = ps.id)
     )a ON (a.id=p.id)
    
  • unless you're intending to return the fields as special_price.price and date.date why not alias the names inside the subquery? e.g.

    SELECT p.*, p.name AS  name, p.image, p.price, ps.*
    FROM product p
    LEFT JOIN
       (SELECT
          psi.price as special_price, psi.date as my_date 
        FROM product_special psi
        WHERE 
          p.id = psi.id AND
          psi.date < NOW()
        ORDER BY psi.priority ASC, LIMIT 1
       ) AS ps ON
      p.id = ps.id
    

    Does your query language have a FIRST() aggregate function? Not sure if you could make the PK of product_special a composite between id and priority (both ASC sort) and change the ORDER clause to GROUP BY id, psi.priority

    you MIGHT be able to remove the ORDER BY clause entirely and use HAVING MIN(psi.priority)

  • Note that the "cross apply" mechanism from SQL Server would solve this, but it isn't available in PostgreSQL. Basically, it was their solution for how to pass parameters (which tend to be references to columns external to the current table expression) to functions called as table expressions in the FROM clause. But it turned out to be useful for all kinds of situations where you want to avoid another level of subquery nesting or moving things from the FROM clause to the SELECT clause. PostgreSQL made it possible to do this by making kind of an exception -- you can pass parameters like that if the expression is a simple function call but not strictly speaking an embedded SELECT. So

    left join highestPriorityProductSpecial(p.id) on true

    is ok, but not

    left join (select * from product_special ps where ps.id = p.id order by priority desc limit 1) on true

    even though the definition of the function is precisely that.

    So, that is in fact a handy solution (in 9.1 at least): make a function to extract your highest priority row by doing the limit inside the function.

    But functions have the drawback that the query plan will not show what is going on inside them and I believe it will always choose a nested loop join, even when that might not be best.

    `cross apply` **is** available in Postgres starting with 9.3 (released in 2013) but they chose to adhere to the SQL standard and use the standard `lateral` operator. In your second query replace `left join` with `left join lateral`

  • Try the following SQL command:

    SELECT p.name,p.image,p.price,pss.price,pss.date
    FROM Product p OUTER APPLY(SELECT TOP(1)* 
    FROM ProductSpecial ps
    WHERE p.Id = ps.Id ORDER BY ps.priority )as pss
    

    may you please add more information to your answer

    The code in question uses `LIMIT` and is not tagged with a DBMS (so it could be MySQL or Postgres or SQLite or possibly some other dbms). The code in the answer uses `OUTER APPLY` and `TOP` so it it will work in SQL Server only (and Sybase) which do not have `LIMIT`.

    This one is applicable for sql server only for other databases we can use inner query within the select statement.

    In Postgres there isn't `OUTER APPLY`, but there is LATERAL, which should be equivalent. An example using it: https://stackoverflow.com/a/47926042/4850646

  • Inspired by dezso's answer https://dba.stackexchange.com/a/222471/127433 I'm solving the problem in PostgreSQL using arrays, like this:

    SELECT 
       *, 
       p.name AS name, 
       p.image, 
       p.price,
       ( 
           SELECT ARRAY[ps.price, ps.date]
           FROM product_special ps 
           WHERE p.id = ps.id
             AND ps.date < NOW() 
           ORDER BY ps.priority ASC, LIMIT 1
       ) AS special_price_and_date
    FROM product p LEFT JOIN product_special ps ON (p.id = ps.id)
    

    Admittedly it's still just one column, but in my code, I can easily access the two values. Hope it works for you as well.

  • I just want to put this here for last resort, for everyone who uses database engine that does not support one or more of the other answers...

    You can use something like:

    SELECT (col1 || col2) as col3 
    

    (With separator, or formatting col1 and col2 to specific length.) And later draw your data using sub-strings.

    I hope someone finds it useful.

  • In DB2 for z/OS, use pack and unpack functions to return multiple columns in a subselect.

    SELECT 
       *, 
       p.name AS name, 
       p.image, 
       p.price,
        unpack((select PACK (CCSID 1028,
                   ps.price,
                   ps.date)
             FROM product_special ps 
           WHERE p.id = ps.id
             AND ps.date < NOW() 
           ORDER BY ps.priority ASC, LIMIT 1)) .* AS (SPECIAL_PRICE double, DATE date)
    FROM product p LEFT JOIN product_special ps ON (p.id = ps.id);
    

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Content dated before 6/26/2020 9:53 AM