谁能解释一下这条语句的执行过程

select * from customers c where 3<=(select count(*)  from order1 o  where c.ID=o.cus_ID)
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先执行select count(*)  from order1 o  where c.ID=o.cus_ID
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先看看数据库基础吧.这个挺简单的.
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ls的ls真是精辟啊
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customers c
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