SQL Cheat Sheet: High‑Frequency String, Date, and Math Functions Ready for Immediate Use

String Functions

LEN() – returns the length of a string. SELECT LEN('SQL小白实战') AS StrLength; SUBSTRING() – extracts a substring from a specified start position with a given length. SELECT SUBSTRING('13800138000',1,3) AS 手机号前三位; REPLACE() – replaces occurrences of a substring with another string. SELECT REPLACE('测试文本','测试','实战') AS NewText; UPPER() and LOWER() – convert a string to upper‑case or lower‑case. SELECT UPPER('sql'), LOWER('SQL'); LTRIM() and RTRIM() – remove leading or trailing spaces.

SELECT LTRIM(' 左空格'), RTRIM('右空格 ');

Date & Time Functions

GETDATE() – returns the current timestamp. SELECT GETDATE() AS NowTime; DATEPART() – extracts a specific part (year, month, day, hour, etc.) from a date.

SELECT DATEPART(YEAR,GETDATE()) AS 年,
       DATEPART(MONTH,GETDATE()) AS 月,
       DATEPART(DAY,GETDATE()) AS 日;

DATEADD() – adds or subtracts a specified interval to a date. SELECT DATEADD(DAY,3,GETDATE()) AS AfterThreeDay; DATEDIFF() – calculates the difference between two dates in a specified unit.

SELECT DATEDIFF(DAY,'2025-01-01',GETDATE()) AS DiffDay;

Math Functions

ROUND() – rounds a numeric value to a given number of decimal places. SELECT ROUND(99.876,2) AS 保留两位小数; CEILING() and FLOOR() – round a number up or down to the nearest integer. SELECT CEILING(10.1), FLOOR(10.9); ABS() – returns the absolute value of a number.

SELECT ABS(-99.9) AS 绝对值;

Conversion Functions

CAST() and CONVERT() – change a value from one data type to another.

-- Number to string
SELECT CAST(123 AS VARCHAR(20)) AS StrNum;

-- Date formatting (ISO 8601)
SELECT CONVERT(VARCHAR(10),GETDATE(),23) AS 标准日期;