CASE

SELECT
    product_name,
    CASE
WHEN product_type = '衣服' THEN
    'A:' || product_type
WHEN product_type = '办公用品' THEN
    'B:' || product_type
WHEN product_type = '厨房用具' THEN
    'C:' || product_type
ELSE
    NULL
END AS abc_product_type
FROM
    Product;

ELSE字句其实是可以不写的，默认的就是ELSE NULL。

使用CASE表达式进行行列转换

SELECT
    sum(
        CASE
        WHEN product_type = '衣服' THEN
            sale_price
        ELSE
            0
        END
    ) AS sum_price_clothes,
    sum(
        CASE
        WHEN product_type = '厨房用具' THEN
            sale_price
        ELSE
            0
        END
    ) AS sum_price_kitchen,
    sum(
        CASE
        WHEN product_type = '办公用品' THEN
            sale_price
        ELSE
            0
        END
    ) AS sum_price_office
FROM
    product;