SQL中的条件表达式

Question

I am using MYSQL and PHP. I have a table called item. Two of its attributes are price and discounted price.I have SELECT statement as below:

$sql = 
'SELECT C.cart_id,I.item_id,C.quantity, I.discounted_price,I.price
FROM cart C, item I
WHERE I.item_id = C.item_id';

I want to modify this sql statement and include a conditional expression such that: if discounted_price is 0, it selects price else, discounted_price is selected. Can conditional expression be used in an SQL statement? Thanks! :)

Answer 1

Try this:

SELECT
    C.cart_id,
    I.item_id,
    C.quantity,
    COALESCE(I.discounted_price, I.price) AS price
FROM
    cart C,
    item I
WHERE
    I.item_id = C.item_id

COALESCE selects the first non null value from a list of arguments. That is, when discount_price is NULL, it returns price.

If your 'null' value is 0.00, you can use this:

SELECT
    C.cart_id,
    I.item_id,
    C.quantity,
    IF(I.discounted_price > 0, I.discounted_price, I.price) AS price
    ...

That is, if discounted price is greater than zero, use that, otherwise use the normal price.

Answer 2

In SQL Server that would be:

SELECT C.cart_id,I.item_id,C.quantity, CASE WHEN I.discounted_price IS NULL THEN I.price ELSE I.discounted_price END AS price
FROM cart C, item I
WHERE I.item_id = C.item_id

You could give that a try in mysql.

Answer 3

Something like

SELECT C.cart_id,I.item_id,C.quantity, 
    IfNull(I.discounted_price,I.price) as Price
FROM cart C, item I
WHERE I.item_id = C.item_id

Use IsNull if you are using MSSQL