sql - SQL在存储过程中使用if语句更新表

Question

我有一个学生表，有两个字段，StudentName 和 Grade。我正在尝试编写一个存储过程来更新成绩。如果学生有 A，我想把它改成 B。如果他们有 B，我想把它改成 A。如果他们还有其他东西，我想不管它。这是我最好的尝试

create procedure sp_changegrades
if Grade = 'A' update Students set Grade = 'B'

else if Grade = 'B' update Students set Grade = 'A'

Answer 1

只需使用CASE

UPDATE Students
SET Grade = 
(
   CASE WHEN Grade = 'A' THEN 'B'
        WHEN Grade = 'B' THEN 'A'
        ELSE Grade     -- "If they have anything else I want to leave it alone."
    END
)

或者

UPDATE Students
SET Grade = 
(
   CASE WHEN Grade = 'A' 
        THEN 'B'
        ELSE 'A'
    END
)
WHERE Grade IN ('A','B')

Answer 2

你可以这样写。在此解决方案中，您定义join部分更新然后更新的规则。

create procedure sp_changegrades
as
begin
    update Students set
        Grade = G.Grade_New
    from Students as S
        inner join (values
            ('A', 'B'),
            ('B', 'A')
        ) as G(Grade_Old, Grade_New) on G.Grade_Old = S.Grade 
end

或者你可以使用case

create procedure sp_changegrades
as
begin
    update Students set
        Grade = 
           case Grade
               when 'A' then 'B'
               when 'B' then 'A'
               else Grade
           end
end

Answer 3

您可以使用 Case 语句，并添加 where 子句，以便仅更新相关行。

UPDATE Students
SET Grade = 
(
   CASE WHEN Grade = 'A' THEN 'B'
        WHEN Grade = 'B' THEN 'A'
        ELSE Grade     -- "Included for Completeness, should never be utilized."
    END
)
WHERE Grade in ('A','B')