sql 判断表数据是否存在另一个表中

目前业务列表中需要展示一个合同在另一个表是否存在的状态

select *,
	case when (select count(*) from b where id = a.id) > 0 
		then 1 
	else 0 
	end as flag 
from a
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如果想a表和b表的字段和id这两列都一样，才将flag显示为1的话

select *, 
	case when (select count(*) from b where id = a.id and 字段 = a.字段) > 0 
		then 1 
	else 0 
	end as flag 
from a
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