t sql"select case"vs"if ... else"和关于"开始"的解释

Question

我对t sql的经验很少,我必须写一个存储的.

这是我存储的:

USE myDatabase
GO

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

CREATE PROCEDURE [dbo].[myStored]
(
@myPar1 INT,
@myPar2 SMALLDATETIME
)

AS

BEGIN
  SET NOCOUNT ON

  IF EXISTS (
            SELECT 
            1 

            FROM 
            myTable1 

            WHERE 
            myPar1 = @myPar1
            AND myPar2 = @myPar2
            )

    BEGIN
      DELETE FROM  
      myTable1  

      WHERE 
      myPar1 = @myPar1
      AND myPar2 = @myPar2
    END

  ELSE 

    IF EXISTS (
              SELECT 
              1 

              FROM 
              myTable2 

              WHERE 
              myPar2 = @myPar2
              )

      BEGIN
        INSERT INTO  
        myTable1
        (myField1, myField2, myField3, myField4)

        VALUES
        (@myPar1, @myPar2, '', 1)
      END    

    ELSE

      IF EXISTS (
                SELECT 
                1 

                FROM 
                myTable3 

                WHERE 
                myPar2 = @myPar2
                )

        BEGIN
          INSERT INTO  
          myTable1
          (myField1, myField2, myField3, myField4)

          VALUES
          (@myPar1, @myPar2, '', 1)
        END
END

这些是我的问题:

1 - 是否存在宏观错误？

2 - 有人建议使用"SELECT CASE"其他人使用"IF ... ELSE",有什么区别？什么是我存储的最佳选择？

3 - 我不确定使用"BEGIN ... END"语句,特别是结合"IF ... ELSE"语句.这是什么意思？是否有必要在"IF ... ELSE"声明中加入"BEGIN ... END"？还用于执行单个指令？

Answer 1

对于单个IF声明

IF (Some Condition)     --<-- If condition is true control will get inside the 
 BEGIN                     -- BEGIN ..END Block and execute the Code inisde
   /* Your Code Here*/
 END

所有单一的IF语句都将独立检查条件.

ONE IF with ONE ELSE

IF (Some Condition)     --<-- If condition is true control will get inside the 
 BEGIN                     -- BEGIN ..END Block and execute the Code inisde
   /* Your Code Here*/     -- IF not true control will jump to Else block
 END
ELSE       --<-- You dont mention any condition here
 BEGIN                    
   /* Your Code Here*/
 END

只有一个代码块将执行IF true然后第一个块的Otherwsie ELSE代码块.

多个IF和ELSE

IF (Some Condition)     --<--1) If condition is true control will get inside the 
  BEGIN                     -- BEGIN ..END Block and execute the Code inisde
    /* Your Code Here*/     -- IF not true control will check next ELSE IF Blocl
  END
ELSE IF (Some Condition)       --<--2) This Condition will be checked
  BEGIN                    
      /* Your Code Here*/
  END
ELSE IF (Some Condition)       --<--3) This Condition will be checked
  BEGIN                    
     /* Your Code Here*/
  END
ELSE                --<-- No condition is given here Executes if non of
  BEGIN                --the previous IFs were true just like a Default value   
     /* Your Code Here*/
  END

只有第一个代码块才会被执行WHERE如果条件为真则休息将被忽略.

开始..END块

在任何IF,ELSE IF或ELSE之后如果你执行多个语句,你必须将它们包装在一个BEGIN..END块中.如果您只执行一个语句,则没有必要但是始终使用BEGIN END块是一个好习惯,这样可以更容易地读取代码.

你的程序

我已经取出ELSE语句使每个IF语句都检查给定的条件独立现在你有一些想法如何处理IF和ELSE所以请自己动手,因为我不知道你在这里尝试应用什么逻辑.

CREATE PROCEDURE [dbo].[myStored]
(
@myPar1 INT,
@myPar2 SMALLDATETIME
)

AS

BEGIN
  SET NOCOUNT ON

  IF EXISTS (SELECT 1 FROM myTable1 WHERE myPar1 = @myPar1
            AND myPar2 = @myPar2)
    BEGIN
      DELETE FROM  myTable1  
      WHERE myPar1 = @myPar1 AND myPar2 = @myPar2
    END


    IF EXISTS (SELECT 1 FROM myTable2 WHERE myPar2 = @myPar2)
      BEGIN
         INSERT INTO  myTable1(myField1, myField2, myField3, myField4)
         VALUES(@myPar1, @myPar2, '', 1)
      END    


      IF EXISTS (SELECT 1 FROM myTable3  WHERE myPar2 = @myPar2)
        BEGIN
           INSERT INTO  myTable1(myField1, myField2, myField3, myField4)
           VALUES(@myPar1, @myPar2, '', 1)
        END
END