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SQL Server遍历表的几种方法

  在数据库开发过程中,我们经常会碰到要遍历数据表的情形,一提到遍历表,我们第一印象可能就想到使用游标,使用游标虽然直观易懂,但是它不符合面向集合操作的原则,而且性能也比面向集合低。当然,从面向集合操作的角度出发,也有两种方法可以进行遍历表的操作,总结起来,遍历表有下面几种方法。

  1. 使用游标
  2. 使用表变量
  3. 使用临时表

我的需求是:针对HR.Employees表,新增一列fullname,并取值firstname+lastname。

这个需求本来可以一条sql语句搞定,如下代码所示。但是为了演示表的遍历,我还是使用了这三种方式来实现一下。

USE TSQLFundamentals2008;GOUPDATE HR.Employees SET fullname= firstname+ +lastname;

使用游标

  使用游标的代码比较繁琐,概括起来主要有以下几个步骤,声明游标,打开游标,使用游标,关闭游标和释放游标。示例代码如下。

 

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USE TSQLFundamentals2008;GOSELECT * FROM HR.Employees;-- 需求是,新增一列fullname,取值firstname+lastnameALTER TABLE HR.Employees ADD fullname NVARCHAR(30) NULL;GO-- 方法1:游标-- 声明变量DECLARE    @empid AS INT,    @firstname AS NVARCHAR(10),    @lastname AS NVARCHAR(20);    -- 声明游标DECLARE C_Employees CURSOR FAST_FORWARD FOR    SELECT empid,firstname,lastname     FROM HR.Employees    ORDER BY empid;    OPEN C_Employees;-- 取第一条记录FETCH NEXT FROM C_Employees INTO @empid,@firstname,@lastname;WHILE @@FETCH_STATUS=0BEGIN    -- 操作    UPDATE HR.Employees SET fullname= @firstname+ +@lastname WHERE empid=@empid;        -- 取下一条记录    FETCH NEXT FROM C_Employees INTO @empid,@firstname,@lastname;END-- 关闭游标CLOSE C_Employees;-- 释放游标DEALLOCATE C_Employees;-- 方法2:使用表变量-- 声明表变量DECLARE @temp TABLE(    empid INT,    firstname NVARCHAR(10),    lastname NVARCHAR(20));-- 将源表中的数据插入到表变量中INSERT INTO @temp(empid, firstname, lastname )SELECT empid,firstname,lastname FROM HR.EmployeesORDER BY empid;-- 声明变量DECLARE    @empid AS INT,    @firstname AS NVARCHAR(10),    @lastname AS NVARCHAR(20);    WHILE EXISTS(SELECT empid FROM @temp)BEGIN    -- 也可以使用top 1    SET ROWCOUNT 1    SELECT @empid= empid, @firstname= firstname,@lastname= lastname FROM @temp;    UPDATE HR.Employees SET fullname= @firstname+ +@lastname WHERE empid=@empid;    SET ROWCOUNT 0        DELETE FROM @temp WHERE empid=@empid;END-- 方法3:使用临时表-- 创建临时表IF OBJECT_ID(tempdb.dbo.#tempemployees,U) IS NOT NULL DROP TABLE dbo.#tempemployees;GOSELECT empid,firstname,lastname INTO dbo.#tempemployeesFROM HR.EmployeesORDER BY empid;--SELECT * FROM dbo.#tempemployees;-- 声明变量DECLARE    @empid AS INT,    @firstname AS NVARCHAR(10),    @lastname AS NVARCHAR(20);    WHILE EXISTS(SELECT empid FROM dbo.#tempemployees)BEGIN    -- 也可以使用top 1    SET ROWCOUNT 1    SELECT @empid= empid, @firstname= firstname,@lastname= lastname FROM dbo.#tempemployees;    UPDATE HR.Employees SET fullname= @firstname+ +@lastname WHERE empid=@empid;    SET ROWCOUNT 0        DELETE FROM dbo.#tempemployees WHERE empid=@empid;END--UPDATE HR.Employees SET fullname=NULL;--SELECT * FROM HR.Employees;--USE TSQLFundamentals2008;--GO--UPDATE HR.Employees SET fullname= firstname+‘ ‘+lastname;
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SQL Server遍历表的几种方法