SELECT COALESCE(field1, field2, field3) AS first_non_null_value FROM example_table; 这个查询会检查 field1,如果 field1 是非空的,就返回 field1 的值;如果 field1 是空的,就检查 field2,以此类推,直到找到第一个非空的值。 使用CASE WHEN 语句 CASE WHEN 语句提供了更灵活的条件判断逻辑。同样的...
SELECT first_name, COALESCE(middle_name,'') AS middle_name, last_name, CONCAT_WS(' ', first_name, COALESCE(middle_name,''), last_name) AS full_name FROM employees; In this query, we use the coalesce function to handle the cases where the value of the “middle_name” column is NU...
SELECT COUNT(DECODE(DEPT_NO,0020,'X',NULL))D0020_COUNT, COUNT(DECODE(DEPT_NO,0030,'X',NULL))D0030_COUNT, SUM(DECODE(DEPT_NO,0020,SAL,NULL)) D0020_SAL, SUM(DECODE(DEPT_NO,0030,SAL,NULL)) D0030_SALFROM EMPWHERE ENAME LIKE 'SMITH%';类似的,DECODE函数也可以运用于GROUP BY 和ORDER ...
(select *,lag(city) over (order by id) as ncity,lag(province) over (order by id) as npr...
当用户输入手机号或email登录时,程序会判定输入信息的存在与否性,存在则走登录,不存在则走注册。而保证唯一性就不仅仅需要在程序端做判断,还需要MySQL的唯一索引去做最后一道防线。那么唯一索引在一些业务中使用,如果唯一索引字段中默认值设置为了null,会造成什么后果呢?
테이블에 전체 텍스트 인덱스를 만드는 경우 테이블에는 하나의 고유한 non-null 열이 있어야 합니다. char, varchar, nchar, nvarchar, text, ntext, image, xml, varbinary 및 varbinary(max) 유형의 열에 전체 텍스...
7: create nonclustered index #table_index2 on #table (Salary) include (Department,empid ) 8: insert into #table select S.empid,S.empname,T.deptname,S.salary from Employees s 9: inner join Departments T ON S.deptid =T.deptid
SELECT CONSTRAINT INTERSECT SESSION_USER CONTAINS INTO SET CONTAINSTABLE IS SETUSER CONTINUE JOIN SHUTDOWN CONVERT KEY SOME CREATE KILL STATISTICS CROSS LEFT SYSTEM_USER CURRENT LIKE TABLE CURRENT_DATE LINENO TABLESAMPLE CURRENT_TIME LOAD TEXTSIZE ...
SQL基础学习笔记 一、建库建表 1、检查数据库名是否存在 如果需要创建数据库,可能会出现数据库名字重名的现象,我们可以使用如下代码查询数据库名是否存在,存在则删除此数据库。 --删除数据库 if exists(select * from sys.databases where name = 
This indicates that the commit of the inner transaction from the first EXECUTE statement of TransProc was overridden by the subsequent roll back of the outer transaction. */ SELECT * FROM TestTrans; GO 當外部交易啟用時,資料庫引擎會忽略內部交易的認可。 交易是認可或復原,取決於...