使用子查询优化 MySQL SELECT 查询

Question

Hi is it possible to optimize the SELECT query below? The query itself is working but when we are querying large data we are encountering a message in php which is "Maximum execution time of 30 seconds exceeded". I reduced the columns in the query up to Topping3 but I am querying up to Topping15 column.

SELECT 
    itemID,
    itemName,
    Topping1,
    (SELECT DISTINCT Description FROM items WHERE PLU = a.Topping1 AND ClientID = 1679) AS Top1_desc,
    Topping2,
    (SELECT DISTINCT Description FROM items WHERE PLU = a.Topping2 AND ClientID = 1679) AS Top2_desc,
    Topping3,
    (SELECT DISTINCT Description FROM items WHERE PLU = a.Topping2 AND ClientID = 1679) AS Top3_desc,
FROM
    items a
WHERE
    ...

Current data on items table

--------------------------------------------------------------------
| itemID | itemName | Description | Topping1 | Topping2 | Topping3 |
--------------------------------------------------------------------
|    1   |   HAM1   |  Hamburger  |   ONI1   |   TOMO1  |          |
--------------------------------------------------------------------
|    2   |   ONI1   |    Onion    |          |          |          |
--------------------------------------------------------------------
|    3   |   TOMO1  |   Tomato    |          |          |          |
--------------------------------------------------------------------

and this is the expected result

--------------------------------------------------------------------------------------------------------
| itemID | itemName | Description | Topping1 | Top1_desc | Topping2 | Top2_desc | Topping3 | Top3_desc |
--------------------------------------------------------------------------------------------------------
|    1   |   HAM1   |  Hamburger  |   ONI1   |   Onion   |   TOMO1  |  Tomato   |          |          |
--------------------------------------------------------------------------------------------------------
|    2   |   ONI1   |    Onion    |          |           |          |           |          |          |
--------------------------------------------------------------------------------------------------------
|    3   |   TOMO1  |   Tomato    |          |           |          |           |          |          |
--------------------------------------------------------------------------------------------------------

Answer 1

This should be fast, unless there are a lot of entries with the same client ID. You could add LIMIT 1 after all subqueries, ie: (SELECT DISTINCT Description FROM items WHERE PLU = a.Topping1 AND ClientID = 1679 LIMIT 1) etc.

I suspect however that it is an index problem. Are the fields ClientID and PLU indexed?

EDIT: Alternative for your query:

SELECT 
    itemID,
    itemName,
    Topping1,
    t1.Description AS Top1_desc,
    Topping2,
    t2.Description AS Top2_desc,
    Topping3,
    t3.Description AS Top3_desc,
FROM
    items a
LEFT JOIN
    items t1 ON t1.PLU=a.Topping1 AND t1.ClientID = 1679
LEFT JOIN
    items t2 ON t2.PLU=a.Topping2 AND t2.ClientID = 1679
LEFT JOIN
    items t3 ON t3.PLU=a.Topping3 AND t3.ClientID = 1679
WHERE
    ...
GROUP BY
    a.itemID

Fields itemID , PLU and ClientID need indexes.