MySQL 函数
Aggregate Functions
COUNT()
" 返回表的记录数 SELECT COUNT(*) FROM table_name " 返回 column_name 值的数量，NULL 值不会被统计 SELECT COUNT(column_name) FROM table_name " 返回指定列的不同（独特）值的个数 SELECT COUNT(DISTINCT column_name) FROM table_name
String Functions
LENGTH()
返回字符串的长度
SELECT LENGTH(column_name) FROM table_name