判断字段长度函数length

SELECT name,capital
FROM world
WHERE LENGTH(name)=LENGTH(capital)

 

求字串，注意分做字串和右字串

LEFT(ARG,LENGTH)、RIGHT(ARG,LENGTH)

LEFT、RIGHT函数返回ARG最左边、右边的LENGTH个字符串，ARG可以是CHAR或BINARY STRING。

SELECT name,capital
FROM world
WHERE LEFT(name,1)=LEFT(capital,1) and name!=capital

 

保留小数点ROUND() 语法

SELECT ROUND(column_name,count)

FROM table_name

column_name字段，保留count个小数个数