MySQL：如何从另一个表的列中选择数据

Question

I have the following select statement:

  SELECT
    items.id,
    items.name AS item_name,
    items.tobuy,
    items.list_id,
    items.note,
    items.unit,
    MIN(NULLIF(packages.ppu, 0)) AS mppu,
    packages.price AS mprice,
    items._deleted
  FROM
    items
  INNER JOIN
    lists ON lists.id = items.list_id
  LEFT JOIN
    packages ON items.id = packages.item_id
  WHERE
    lists.user_id = 1 AND
    items._deleted = '0'
  GROUP BY
    items.id
  ORDER BY
    tobuy DESC,
    item_name

But what I really want is the price to come from the package that has the minimum ppu (which is not necessarily the package with the minimum price).

Any ideas?

Sample Records:

Table: items:

id,  name,  tobuy, list_id, note, unit, _deleted
95,  test1, 1,     1,       null, null, 0
69,  test2, 1,     1,       null, null, 0
194, test3, 1,     1,       null, null, 0
162, test4, 1,     1,       null, null, 0

Table: lists:

id, name,  user_id
1,  list1, 1

Table: packages:

id,  item_id, price, ppu
392, 95,      0,     0
117, 95,      13.49, 0.078
391, 95,      0,     0
386, 69,      0,     0
387, 69,      0,     0
388, 69,      0,     0
368, 194,     4.58,  0.138
18,  194,     3.38,  0.177
17,  194,     3.88,  0.144

The results should be four items with the following information:

id,  item_name,  tobuy, list_id, note, unit, mppu,  mprice, _deleted
95,  test1,      1,     1,       null, null, 0.078, 13.49,  0
69,  test2,      1,     1,       null, null, 0,     0,      0
194, test3,      1,     1,       null, null, 0.138, 4.58,   0
162, test4,      1,     1,       null, null, 0,     0,      0

Notice that item 162 doesn't have any corresponding packages, but it still shows up in the list. This is the reason for the "LEFT JOIN"

BTW, "mppu" stands for "minimum price per unit"

Answer 1

can you try for the 2nd left: packages ON ( packages.item_id = items.id and select min(packages.ppu) from packages where item_id=item.id)

also make sure to change group by to items.id not tested

Answer 2

With mysql this is surprisingly simplex.

Order by ppn and move the group by to an outer query

SELECT * FROM (
  SELECT
    items.id,
    items.name AS item_name,
    items.tobuy,
    items.list_id,
    items.note,
    items.unit,
    MIN(NULLIF(packages.ppu, 0)) AS mppu,
    packages.price AS mprice,
    items._deleted
  FROM items
  INNER JOIN lists
    ON lists.id = items.list_id
    AND lists.user_id = 1
  LEFT JOIN packages
    ON items.id = packages.item_id
  WHERE items._deleted = '0'
  ORDER BY
    ppm,
    tobuy DESC,
    item_name) x
GROUP BY
  id

Answer 3

The following works to get the results that I was looking for.

SELECT * FROM (  
  SELECT
    items.id,
    items.name AS item_name,
    items.tobuy,
    items.list_id,
    items.note,
    items.unit,
    NULLIF(packages.ppu, 0) AS mppu,
    packages.price AS mprice,
    items._deleted
  FROM
    items
  INNER JOIN
    lists ON lists.id = items.list_id
    AND lists.user_id = 1 
  LEFT JOIN
    packages ON packages.item_id = items.id
  WHERE
    items._deleted = '0'
  ORDER BY
    tobuy DESC,
    item_name,
    IFNULL(mppu, 999999)) x
GROUP BY
  x.id
ORDER BY
  tobuy DESC,
  item_name

Thanks to everyone for the input to get me through this.