select  函数名(实参列表) 【from 表】;

SELECT LENGTH(‘join‘);
SELECT LENGTH(‘张三丰taijiquan‘);

SELECT CONCAT(last_name,‘ ‘,first_name) FROM employees;

-- substr(string, start, length)
-- 截取从指定索引处后面所有的字符
SELECT SUBSTR(‘李莫愁爱上了陆展元‘,7) out_put;

-- 截取从指定索引处指定字符长度的字符
SELECT SUBSTR(‘李莫愁爱上了陆展元‘,1,3) out_put;

-- 案例：姓名中首字母大写，其他字符小写然后后用_拼接；
SELECT CONCAT(UPPER(SUBSTR(last_name,1,1)),‘_‘,LOWER(SUBSTR(last_name,2))) AS out_put
FROM employees;

SELECT INSTR(‘下次是哪次‘,‘哪次‘) AS out_put;
SELECT INSTR(‘下次是哪次‘,‘shi哪次‘) AS out_put;

SELECT TRIM(‘          改天是哪天          ‘) AS out_put;

-- 移除指定的首尾字符,备注：前面的‘aa’是一个字符,如果后面是‘aaaaa改天是哪天aaaaa‘,截取后‘a改天是哪天a‘
SELECT TRIM(‘a‘ FROM ‘aaaa改天是哪天aaaa‘) AS out_put;

SELECT UPPER(last_name) FROM employees;

SELECT LOWER(last_name) FROM employees;

-- 案例：姓大写，名小写
SELECT CONCAT(UPPER(last_name),LOWER(first_name)) as 姓名 FROM employees;

SELECT LPAD(‘以后是多后‘,10,‘*‘);
SELECT LPAD(‘以后是多后‘,2,‘*‘);