coalesce 函數可以接受多個參數，將會返回這些參數中第一個非NULL的值，若提供的參數全部為NULL，則返回NULL
ifnull 函數和coalesce功能一樣，只是只可以接受兩個參數
if  函數接受三個參數，實現類似于三元判斷符(?:)的功能，即第一個參數不為NULL且不為0時，返回第二個參數，否則返回第三個參數

復制代碼 代碼如下:

SELECT a.*,coalesce(t.cous,0) as count FROM brand as a
left join (select brandid as bid,count(1) as cous from shopbrand group by brandid) t on a.brandid=t.bid
ORDER BY count DESC LIMIT 0,20