mysql select语句中的回退条件

Question

I want to put a condition into MySql statement. If that is even possible?

This is my current mysql select statement

SELECT *
      FROM `validproduct`
      WHERE `visible` = 1
      AND `typeid` = 2
      AND `sizeid` = 3
      GROUP BY `productid`

So at the moment, I am selecting products with sizeid = 3. What I want to do is to select products with size = 1, and if that doesn't exist, then select sizeid = 3.

My validproducts table contains a list of products with up to 6 different sizes associated with it (which in turn have different pricing). I want to return all of my products, with the lowest sizeid (which will also have the lowest price available for that product). Not all products have a sizeid 1.

So my return data will be something like product A (sizeid 1, $x); product B (sizeid 3, $y) etc

Answer 1

Here is a way to do it:

SELECT 
  vp1.* 
FROM 
  validproduct vp1 
INNER JOIN
  (
    SELECT 
      productid, MIN(sizeid) as minsize 
    FROM 
      validproduct 
    WHERE 
      visible = TRUE AND
      typeid = 2
    GROUP BY
    productid
  ) AS vp2
ON 
  vp1.productid = vp2.productid AND 
  vp1.sizeid = vp2.minsize AND 
  vp1.visible = TRUE AND
  vp1.typeid = 2
GROUP BY 
  vp1.productid;

Explanation:

The outer query is actually what you already have. The inner sub-query finds the minimum sizeid for every product. By joining the two you get what you want.

PLZ SEE THE FIDDLE

Answer 2

It's a bit weird, but OK I guess...

SELECT * FROM ints;
+---+
| i |
+---+
| 0 |
| 1 |
| 2 |
| 3 |
| 4 |
| 5 |
| 6 |
| 7 |
| 8 |
| 9 |
+---+

SELECT COALESCE(i2.i,i1.i)m FROM ints i1 LEFT JOIN ints i2 ON i2.i = 3 WHERE i1.i = 1;
+------+
| m    |
+------+
|    3 |
+------+

SELECT COALESCE(i2.i,i1.i)m FROM ints i1 LEFT JOIN ints i2 ON i2.i = 11 WHERE i1.i = 1;
+------+
| m    |
+------+
|    1 |
+------+

or maybe...

SELECT i FROM ints ORDER BY FIELD(i,1,3) DESC LIMIT 1;
+---+
| i |
+---+
| 3 |
+---+