msql 写法 这事布尔表达式写法么？

SELECT
        top 100 *
FROM
       a t0
WHERE
        t0.orderAvlstatus = 'Y' AND t0.orderDelstatus = 'N'
        AND t0.orderBusinesstype = 'I'
        AND t0.orderType = 'CLASS' AND
        ( SELECT COUNT(1) FROM b t1 WHERE t0.orderId = t1.orderId AND t1.productType = 'CLASS' AND t1.oiMainstatus = 'Y') = 0  

查询的时候怎么是=0 false  与括号里的值不相符     =1 true 与括号里的值相符  这事什么写法 少见多怪了 没学过

等于拿COUNT(1)的值去判断