1、字符串函数

测试用例如下：

1.1 CONCAT()

将多个字符串连接成一个字符串。

SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM users;
-- 期望结果：'John Doe', 'Jane Smith', 'Michael Johnson'

1.2 SUBSTRING()

提取子字符串

SELECT SUBSTRING(column_name, start_pos, length) FROM table_name;

SELECT SUBSTRING(first_name, 1, 3) AS short_name FROM users;
-- 期望结果：'Joh', 'Jan', 'Mic'

1.3 UPPER()

将字符串转换为大写

SELECT UPPER(column_name) FROM table_name;

SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM users;
-- 期望结果：'John Doe', 'Jane Smith', 'Michael Johnson'

1.4 LOWER()

将字符串转换为小写

SELECT LOWER(column_name) FROM table_name;

SELECT LOWER(last_name) AS lower_name FROM users;
-- 期望结果：'doe', 'smith', 'johnson'

1.5 LENGTH()

返回字符串的长度

SELECT LENGTH(column_name) FROM table_name;

SELECT LENGTH(first_name) AS name_length FROM users;
-- 期望结果：4, 4, 7

1.6 TRIM()

去除字符串两端的空格或指定字符

SELECT TRIM(' ' FROM column_name) FROM table_name;

SELECT TRIM(' ' FROM ' John Doe ') AS trimmed_name;
-- 期望结果：'John Doe'

1.7 REPLACE()

替换字符串中的字串

SELECT REPLACE(column_name, 'old_string', 'new_string') FROM table_name;

SELECT REPLACE(first_name, 'J', 'M') AS replaced_name FROM users;
-- 期望结果：'Monn', 'Mane', 'Michael'

2、数值函数

测试用例如下：

2.1 SUM()

计算数值列的总和。

SELECT SUM(column_name) FROM table_name;

SELECT SUM(quantity) AS total_quantity FROM sales;
-- 期望结果：23

2.2 AVG()

计算数值列的平均值

SELECT AVG(column_name) FROM table_name;

SELECT AVG(unit_price) AS average_price FROM sales;
-- 期望结果：16.083333

2.3 COUNT()

计算行数或者非NULL值的数量

SELECT COUNT(*) FROM table_name;

SELECT COUNT(*) AS total_records FROM sales;
-- 期望结果：3

2.4 MAX()

返回数值列的最大值

SELECT MAX(column_name) FROM table_name;

SELECT MAX(unit_price) AS max_price FROM sales;
-- 期望结果：20.00

2.5 MIN()

返回数值列的最小值

2.6 ROUND()

对小数四舍五入到指定位数

SELECT ROUND(column_name, decimals) FROM table_name;

SELECT ROUND(unit_price, 1) AS rounded_price FROM sales;

-- 期望结果：15.5, 20.0, 12.8

3、日期函数