Sql 在Where子句中，添加检查参数是否为空字符串

我必须用SQL编写查询，它将返回帐户Id

对于这个查询，我有输入参数，可以是空的

在数据库中，这里是一些字段为null或空的行。所以，若参数为空，我还需要检查字段是否为空

SELECT a.Account_ID   
    FROM Accounts a
    WHERE a.FirstName = @FirstName /*Add check if @FirstName = '' then a.FirstName IS NULL */ 
        AND a.LastName = @LastName 
        AND a.Middle = @MiddleName
        AND a.Email = @Email 
        AND a.Company = @Company

---

如果我想要空参数的特殊行为，我通常会这样做：

SELECT a.Account_ID   
FROM Accounts a
WHERE 
 (
     (@firstname != '' AND a.FirstName = @FirstName) 
     OR (@firstname = '' AND a.FirstName is null)
 )
 AND a.LastName = @LastName AND a.Middle = @MiddleName
 AND a.Email = @Email AND a.Company = @Company

---

我有一个括号中的OR，它处理param=''和param！=''的情况具体而言。

我认为这是您需要的一个条件：

SELECT a.Account_ID   
    FROM Accounts a
    WHERE ((@FirstName='' and a.FirstName is null) or a.FirstName = @FirstName) /*Add check if @FirstName = '' then a.FirstName IS NULL */ 
        AND a.LastName = @LastName 
        AND a.Middle = @MiddleName
        AND a.Email = @Email 
        AND a.Company = @Company

---

不确定我是否理解这个问题，但这有帮助吗

SELECT a.Account_ID   
FROM Accounts a
WHERE ISNULL(a.FirstName,'') = @FirstName 
    AND a.LastName = @LastName 
    AND a.Middle = @MiddleName
    AND a.Email = @Email 
    AND a.Company = @Company

试试这个：

SELECT a.Account_ID   
FROM Accounts a
WHERE 1 = (CASE WHEN @FirstName != '' AND a.FirstName = @FirstName THEN 1 
                WHEN @FirstName = '' AND a.FirstName IS NULL THEN 1 
                ELSE 0 
           END) AND 
      a.LastName = @LastName AND 
      a.Middle = @MiddleName AND 
      a.Email = @Email AND 
      a.Company = @Company

---

case子句也可以在where条件中使用，就像在所选字段列表中一样：

SELECT a.Account_ID   
    FROM Accounts a
    WHERE case when @FirstName = '' then a.FirstName = @FirstName 
               else a.FirstName IS NULL 
          end
      AND a.LastName = @LastName 
      AND a.Middle = @MiddleName
      AND a.Email = @Email 
      AND a.Company = @Company

---

我喜欢你的回答。即使与@gordatron answer几乎相同，我认为check

（@firstname！=''和a.firstname=@firstname）

是正确的necessary@demo我不这么认为。如果

@firstname！=''查询的第一部分返回false，第二部分是executet。如果
（@firstname=''
只执行第一部分。那么@gordatron answer涵盖所有情况？我认为两个查询应该给出相同的结果。