SQL学习笔记：函数

• SQL函数
• AVG
  select AVG(col) AS avgvalue from tablename
  select col2 from tablename where col1>(select AVG(col1) from tablename)
• COUNT 返回值不包含空值
  select COUNT(*) from tablename 返回行数
  select COUNT(DISTINCT col) from tablename 返回列的唯一值
• FIRST 返回第一个值
  select FIRST(col) AS colname from tablename
• LAST 返回最后一个值
  select LAST(col) AS colname from tablename
• MAX 返回最大值
• MIN 返回最小值
• SUM 返回总和
• GROUP BY
  select col1, sum(col2) from tablename GROUP BY col1
• HAVING
  select custom, sum(price) from tablename where custom = 'tom' or custom = 'jerry' group by custom HAVING sum(price)>1000
• UCASE 将字符转化为大写
  select UCASE(col) from tablename
• LCASE 将字符转为小写
• MID 提取字符
  select MID(col, 起始数字, 长度) from tablename
• LEN 提取字符长度
• ROUND 舍入指定位数
  select ROUND(col, 位数) from tablename
• NOW 返回当前时间和日期
  select NOW() from tablename
• FORMAT
  select FORMAT(now(), 'YYYY-MM-DD') from tablename