mysql高级

1.UNSIGNED无符号类型  ZEROFILL用0填充以满足数据长度。

2.bit_count(data):data的二进制数包含1的个数；mysql> select bit_count(3);结果为2

   bit_or():    按位或运算
   bit_and():按位与运算

  例如：

CREATE TABLE t1 (year YEAR(4), month INT(2) UNSIGNED ZEROFILL,

             day INT(2) UNSIGNED ZEROFILL);

INSERT INTO t1 VALUES(2000,1,1),(2000,1,20),(2000,1,30),(2000,2,2),

            (2000,2,23),(2000,2,23);

求上表中：每月的访问量？

SELECT year,month,BIT_COUNT(BIT_OR(1<<day)) AS days FROM t1

       GROUP BY year,month;|

由于1<<day 如果day数据相同，那么运算后的结果是一样的。bit_or运算后对应位1和1或运算结果还是1.这样就达到了去重的效果。

3.用于某个字段组间最大值的行：

SELECT article, dealer, price
FROM   shop s1
WHERE  price=(SELECT MAX(s2.price)
              FROM shop s2
              WHERE s1.article = s2.article);

  或者
   SELECT article,dealer,price from shop 
    where price in (select max(s2.price) from shop s2 group by article);
4.