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一般写子查询的时候:SELECT max(`Posted-date`) as `Postedday` FROM `transaction`where `unique_key` in ("SELECT `unique_key` from `transaction` where `seller`="."'$count_seller'");执行的时候会很慢;
优化:SELECT max(`Posted-date`) as `Postedday` FROM `transaction`where `unique_key` in(SELECT `unique_key` from ("SELECT `unique_key` from `transaction` where `seller`="."'$count_seller'") as tbt));
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