Return a value if no record is found

Question

I have this simple statement that works:

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

If the number does not exist anywhere

Answer 1

Encapsulate the query in a sub-query to transform "no row" to a NULL value.

I tested and verified this with PostgreSQL, SQL Server and MySQL. Also works with SQLite.

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

In Oracle you have to select from the dummy 1-row table DUAL like this:

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

You can do the same in MySQL for compatibility reasons, but you don't have to.
Similar in Firebird:

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

This does it for DB2 (like Sean commented):

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

Alternative with UNION ALL

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

Standard SQL, but I only tested this with Postgres, which evaluates like this:
If a row is found in the first SELECT, it is returned. Postgres stops looking for more rows, as soon as the first is found due to LIMIT 1 (FETCH FIRST 1 ROW ONLY).
The second SELECT is only even executed if the first returns nothing. The data type of the NULL value is determined by the data type of tbl.id automatically.

About the LIMIT clause:

• ANSI/ISO plans for LIMIT standardization?

Answer 2

I use this for MySql

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

Answer 3

To make it more simplier, this should work fine. If you assign this to a variable based on the datatype of your idnumber than you would be able to evaluate whether the value is null or the actual idnumber return.

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

Answer 4

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

Result :

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