Question

这是我的陈述：

SELECT
    a.size AS 'size_id',
    b.size,
    a.stock,
    a.id AS 'stock_id',
    IFNULL(c.quantity,0) as 'cart_qty'
FROM
    stock a
JOIN 
    sizes b ON a.size = b.id
LEFT JOIN
    cart_items c ON a.id = c.stock_id
WHERE
    a.product_id = '{$product['id']}'
AND
    c.cart_id = '$cart_id'
AND
    a.stock > 0
ORDER BY
    b.display_order

在上面，$ cart_id可能为空，或者'cart_items'表中可能没有匹配项。如何改写上述内容，以便如果cart_items中没有匹配项，我仍然可以从与product_id匹配的库存中获得结果？但是如果cart_items中有匹配项，那么我会得到额外的列（cart_qty）？

Answer 1

将谓词移到ON子句：

LEFT JOIN
    cart_items c ON a.id = c.stock_id AND c.cart_id = '$cart_id'

这样，如果cart_items中没有匹配项，您仍会获得与stock匹配的product_id的结果。否则，如果cart_items中有匹配项，则会正确填充其他列cart_qty。

Answer 2

我不确定你能否同时做到这两点。但是，如果$cart_Id is null添加

，请先添加一个

AND ($cart_Id is null or c.cart_id = '$cart_id')

MySQL，条件WHERE子句

2 个答案: