select * from user与select u.id,u.name from user的区别？
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select u.id,u.name from user u比select * from user 效率更高
为什么呢？
原因是第一条语句首先要去检索一下这个表有哪些字段，而第二条则省了这一步操作，所以如果数据量很大的话就不要用select * 了
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