字符函数，日期函数笔记

一、字符函数

字符串拼接：

-- CONCAT拼接多个字段
SELECT CONCAT(FirstName, ' ', LastName) AS FullName FROM Employees;
-- STRING_AGG聚合多行数据
SELECT DepartmentID, STRING_AGG(EmployeeName, ';') AS TeamMembers 
FROM Employees GROUP BY DepartmentID;

格式处理：

-- 去除空格
SELECT LTRIM(RTRIM(Address)) AS CleanAddress FROM Locations;
-- 大小写转换
SELECT UPPER(ProductName) AS ProductCode FROM Products;

二、日期函数

日期转换：

-- 字符转日期
SELECT TO_DATE('2023-12-25', 'YYYY-MM-DD') AS Christmas FROM DUAL;
-- 日期转字符
SELECT TO_CHAR(SYSDATE, 'YYYY"年"MM"月"DD"日"') AS ChineseDate FROM DUAL;

时间计算：

-- 两个日期相差天数
SELECT FLOOR(SYSDATE - HireDate) AS WorkDays FROM Employees;
-- 增加月份
SELECT ADD_MONTHS(SYSDATE, 3) AS ThreeMonthsLater FROM DUAL;

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