DISTINCT() 过滤重复
COUNT()    统计个数
    SELECT COUNT(DISTINCT(cateid)) FROM cs_goods ORDER BY cateid
    SELECT COUNT(*) FROM cs_goods
    
SUM()   求和
       求price列总和
       SELECT SUM(price) FROM cs_goods
       求每个月总销售额
       SELECT SUM(price),SUBSTRING(FROM_UNIXTIME(createtime),1,7) AS ymonth FROM cs_goods GROUP BY ymonth;
       求每天总销售额
       SELECT SUM(price),DATE(FROM_UNIXTIME(createtime)) AS ymonth FROM cs_goods GROUP BY ymonth ORDER BY ymonth DESC;
       求每天销售额大于100的记录
       SELECT SUM(price) AS total,DATE(FROM_UNIXTIME(createtime)) AS ymonth FROM cs_goods GROUP BY ymonth HAVING total>100 ORDER BY ymonth DESC;

AVG()  求平均
       求所有商品平均单价
       SELECT AVG(price) FROM cs_goods;
       求每个分类下商品平均单价
       SELECT AVG(a.price),a.cateid,b.category FROM cs_goods a INNER JOIN cs_category b ON(a.cateid=b.id) GROUP BY cateid;
MAX()  求最大值
       求每个分类下最高单价
       SELECT MAX(a.price),a.cateid,b.category FROM cs_goods a INNER JOIN cs_category b ON(a.cateid=b.id) GROUP BY cateid;
MIN()  求最小值
       求每个分类下最小单价
       SELECT MIX(a.price),a.cateid,b.category FROM cs_goods a INNER JOIN cs_category b ON(a.cateid=b.id) GROUP BY cateid;

数据查询相关函数