Tuesday, July 8, 2008

Execute Dynamic SQL commands in SQL Server




In some applications having hard coded SQL statements is not appealing,
because of the dynamic nature of the queries being issued against the
database server. Because of this sometimes there is a need to dynamically
create
a SQL statement on the fly and then run that command. This can be
done quite simply from the application perspective where the statement is
built on the fly whether you are using ASP.NET, ColdFusion or any other programming
language
. But how do you do this from within a SQL Server stored procedure?

SQL Server offers a few ways of running a dynamically built SQL statement.
These ways are:
1. Writing a query with parameters
2. Using EXEC
3. Using sp_executesql
Writing a query with parameters
This first approach is pretty straight forward if you only need to pass parameters
into
your WHERE clause of your SQL statement. Let's say we need to find all records
from the customers table where City = '
London'. This can be done easily such as
the following example shows.
DECLARE @city varchar(75)
SET @city = '
London'
SELECT * FROM customers WHERE City = @city
2. Using EXEC
With this approach you are building the SQL statement on the fly and can pretty
much do whatever you need to in order to construct the statement. Let'
s say we
want to be able to pass in the column list along with the city.
For this example we want to get columns CustomerID, ContactName and City where
City = 'London'.
As you can see from this example handling the @city value is not at straight
forward, because you also need to define the extra quotes in order to pass a
character value into the query. These extra quotes could also be done within the
statement
, but either way you need to specify the extra single quotes in order for
the query to be built correctly and therefore run.
DECLARE @sqlCommand varchar(1000)
DECLARE @columnList varchar(75)
DECLARE @city varchar(75)
SET @columnList = 'CustomerID, ContactName, City'
SET @city = '''London'''
SET @sqlCommand = 'SELECT ' + @columnList + ' FROM customers WHERE City = ' + @city
EXEC (@sqlCommand)
3. sp_exectesql
With this approach you have the ability to still dynamically build the query,
but you are also able to still use parameters as you could in example 1.
This saves the need to have to deal with the extra quotes to get the query to
build correctly. In addition, with using this approach you can ensure that the
data values being passed into the query are the correct datatypes.
DECLARE @sqlCommand nvarchar(1000)
DECLARE @columnList varchar(75)
DECLARE @city varchar(75)
SET @columnList = 'CustomerID, ContactName, City'
SET @city = 'London'
SET @sqlCommand = 'SELECT ' + @columnList + ' FROM customers WHERE City = @city'
EXECUTE sp_executesql @sqlCommand, N'@city nvarchar(75)', @city = @city

1 comment:

shathar said...

I have a table Person with 2 columns
Gender and City.

i want to retrieve data like the following query outputs using dynamic sql, Please help

select count(Gender) from + @tablename + where Gender='Male' and City= +@cityname

i have to pass table name and city name dynamically and gender column
already contains values