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select * from ( select * from t1 union select * from t1) a,(select * from t1 union select * from t1) b where a.a != 0;
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explain select * from ( select * from t1 union select * from t1) a,(select * from t1 union select * from t1) b where a.a != 0;
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id select_type table type possible_keys key key_len ref rows Extra