MySQL SELECT WHERE OR-仅当第二行为空时才选择一行

Question

I'm joining 2 tables, for example

PRODUCTS (PRODUCT_ID, NAME)

and

PRICE_LEVELS (PRODUCT_ID, PRICE, PRICE_LVL_NAME )

(ofc. it's simplified, there is several joins ).

in the PRICE_LEVELS table, I have some possibilities of price level names, let's say " DEFAULT " and LEVEL1, so we ended up with something like:

PRODUCT_ID | PRICE | PRICE_LVL_NAME
1          | 100   | _DEFAULT_
1          | 50    | LEVEL1
2          | 130   | _DEFAULT_

Both tables are joined in the view.

What I need is to get price, but only once - I mean if there is LEVEL1 defined, pick that one, otherwise pick DEFAULT.

Meantime, I have used GROUP BY and thing seems to work, but I have no idea why (ofc. I've used a lot of test data and it's simply always works, but not sure, how it's reliable).

Let's say our view (combining both tables) has name V_PRODUCTS, so I'm running query:

SELECT * 
FROM `V_PRODUCTS` 
WHERE (PRICE_LVL_NAME = '_DEFAULT_' OR PRICE_LVL_NAME = 'LEVEL1') 
GROUP BY `PRODUCT_ID `;

So the questions are:

1. Why the query above works ? GROUP BY is always choosing LEVEL1, if is available and DEFAULT if not. It's exactly what I need, but need to understand why it's working this way.

2. Is there any way how to do this more explicit in the SQL ?

UPDATE: there is unlimited number of possible levels

Answer 1

Q1: It's not reliable, it's probably based on internal storage and might change at any time.

Q2: Join the table twice using an Outer Join and return the best match using COALESCE:

SELECT ..., COALESCE(pl1.PRICE, pl2.PRICE)
FROM `V_PRODUCTS` as p 
LEFT JOIN PRICE_LEVELS as pl1
  ON p.PRODUCT_ID = pl1.PRODUCT_ID
 and PRICE_LVL_NAME = 'LEVEL1') 
LEFT JOIN PRICE_LEVELS as ply
  ON p.PRODUCT_ID = pl2.PRODUCT_ID
 and PRICE_LVL_NAME = '_DEFAULT_'

Answer 2

I don't have a MySQL to play with right now but you should only have to replace the IS NULL part of this MSSQL to make it work I think.

If you don't need AlphaNum in the levels or can add a level field that is INT where 0 is default and then 1 and so on, you can join the levels to itself to get the max available level to join to the product table.

    SELECT PRODUCTS.*, PriceLvl.*
    FROM [PRODUCTS] LEFT JOIN 
      (SELECT p1.*
       FROM PRICE_LEVELS p1 
            LEFT JOIN PRICE_LEVELS p2 ON 
            (p1.PRODUCT_ID = p2.PRODUCT_ID 
             AND p2.PRICE_LVL > p1.PRICE_LVL)
       WHERE p2.PRICE_LVL IS NULL) PriceLvl 
    ON PRODUCTS.PRODUCT_ID = PriceLvl.PRODUCT_ID

That takes

    PRODUCT_ID  NAME
             1  Tshirts
             2  Pants

and

    PRODUCT_ID  PRICE   PRICE_LVL
             1  10.00   0
             1  15.00   1
             2  40.00   0
             1  20.00   2

and gives

    PRODUCT_ID  NAME    PRODUCT_ID  PRICE   PRICE_LVL
             1  Tshirts          1  20.00   2
             2  Pants            2  40.00   0

Answer 3

I'd assign a numeric value for each level with zero for default, and store it same table.

select t1.price from price_levels t1 
where t1.product_id = @X and price_level = (select max(price_level) 
from price_levels t2 where t1.product_id = t2.product_id)

Answer 4

WHERE price_level IN ('_DEFAULT_', 'mylevel')
ORDER BY
    price_level = '_DEFAULT_'
LIMIT 1

The WHERE will grab 1 or 2 rows.

price_level = '_DEFAULT_' will be 1 for default, otherwise 0. 0 sorts before 1. So, if there is a 'mylevel', it will come first.

LIMIT 1 says to pick the first (or only) row.