Question

I have two product tables. Table A is the “default” table (comes from data feed is automatically updated weekly). Table B is used to manually overwrite the values in Table A and has the same columns as Table plus a few more. Both tables always have the same UniqueID column.

I need a select statement that pulls all the values from A and all the values from B, with B overwriting the values from A UNLESS the value from B is null, = to 0 or equal to 0.00 (column set to integer with 2 decimal points and been filled with “0”).

I tried using a LEFT JOIN but am having trouble with the null and “0” values.

I am using the data in php and can deal with it when I only need to pull in 1 row. I query for each, extract the assoc array for A, filter the B array and then extract it. But that process becomes very clunky when start to pull in 20-200 rows.

Table A has about 400,000 rows and can’t see B getting over 10,000-20,000.

Thanks for your help.

UPDATE: Here is the code I got to work.

SELECT a.UniqueId,
IF(b.Color, b.Color, a.Color) as Color,
IF(b.Price1 = 0, a.Price1, b.Price1) as Price1,
b.SPrice1
FROM productdata a LEFT JOIN product_overrides b ON a.UniqueID = b.UniqueID WHERE a.UniqueID = $prod";


UPDATE 2: So simple is better. IF() addresses both null and zero.

IF(expr1,expr2,expr3)

If expr1 is TRUE (expr1 <> 0 and expr1 <> NULL) then IF() returns expr2; otherwise it returns expr3. IF() returns a numeric or string value, depending on the context in which it is used.

SELECT a.UniqueId,
IF(b.Color, b.Color, a.Color) as Color,
IF(b.Price1, b.Price1, a.Price1) as Price1,
b.SPrice1
FROM productdata a LEFT JOIN product_overrides b ON a.UniqueID = b.UniqueID WHERE a.UniqueID = $prod";


Answer 1

I think this aught to cover all your scenarios:

SELECT 
   COALESCE(b.nullablefield, a.nullablefield) as nullablefield,
   IF(b.intfield = 0, a.intfield, b.intfield) as intfield,
   IF(b.floatfield = 0, a.floatfield, b.floatfield) as floatfield,
   ... etc ...
FROM table_a a
LEFT JOIN table_b b ON a.UniqueID = b.UniqueID