如果未找到记录，则返回一个值

Question

我有这个简单的陈述：

SELECT idnumber FROM dbo.database WHERE number = '9823474'

如果表中的任何位置都不存在该号码，则会失败。我想在这句话中加上一些说法：
如果没有记录被发现，则返回NULL，不得行。

有什么建议吗？

Answer 1

在子查询中封装查询，将“no row”转换为NULL值。

我使用PostgreSQL，SQL Server和MySQL对此进行了测试和验证。也适用于SQLite。

SELECT (SELECT id FROM tbl WHERE id = 9823474) AS id;

在Oracle中，您必须从虚拟1行表DUAL中进行选择，如下所示：

SELECT (SELECT id FROM tbl WHERE id = 9823474) AS id FROM DUAL;

出于兼容性原因，您可以在MySQL中执行相同操作，但您不必这样做 类似于Firebird：

SELECT (SELECT id FROM tbl WHERE id = 9823474) AS id FROM RDB$DATABASE;

这适用于DB2（例如Sean commented）：

SELECT (SELECT id FROM tbl WHERE id = 9823474) AS id FROM SYSIBM.SYSDUMMY1;

替代UNION ALL

SELECT id FROM tbl WHERE id = 9823474
UNION  ALL
SELECT NULL -- FROM DUAL  -- for Oracle
FETCH FIRST 1 ROW ONLY;

标准SQL，但我只用Postgres进行了测试，其评估方式如下：
如果在第一个SELECT中找到一行，则返回该行。由于LIMIT 1（FETCH FIRST 1 ROW ONLY），Postgres会在找到第一行时停止查找更多行。
第二个SELECT只有在第一个没有返回时才执行。 NULL值的数据类型由tbl.id的数据类型自动确定。

关于LIMIT条款：

• ANSI/ISO plans for LIMIT standardization?

Answer 2

为了使它更简单，这应该工作正常。如果根据idnumber的数据类型将此值赋给变量，则可以评估该值是null还是实际的idnumber返回。

SELECT ISNULL(
      (
         SELECT idnumber 
         FROM dbo.database 
         WHERE number = '9823474'
      ), NULL)

Answer 3

Select isnull(sum(Amount),0) as Amt from BeginningBalance where CustomerID = @CustomerID
Union all
Select isnull(sum(Amount),0) as Amt from SalesOrders where CustomerID = @CustomerID
Union all
Select isnull(sum(Amount),0) as Amt from SalesInvoices where CustomerID = @CustomerID
//Data Row Result if no data is present at Beginning Balance Table
// example 

Amt
2000  // amount from sales orders
1000  // amount from sales invoices

---

// if the 1st select statement return no data use this
SELECT (select sum(Amount) from BeginningBalance 
        where CustomerID = @CustomerID) as Amt
Union all
Select sum(Amount) as Amt from SalesOrders where CustomerID = @CustomerID
Union all
Select sum(Amount) as Amt from SalesInvoices where CustomerID = @CustomerID

结果：

Amt
NULL  // amount from BeginningBalance
2000  // amount from sales orders
1000  // amount from sales invoices

Answer 4

我将它用于MySql

SELECT IFNULL(ColumnA,"1") AS ColumnA , COUNT(1) AS Total FROM table 
WHERE ID = 1 LIMIT 0, 1;