How Can I Use a Variable as a Table Name in SQL Queries?

Declare the table name as a variable

Introduction:

Trying to use a variable as a table name in a static query results in the error "Table variable must be declared". This article explores the correct way to populate table names dynamically.

Solution:

For static queries, table names and column names must be static. However, dynamic queries allow for dynamic generation of complete SQL statements, which can be executed using sp_executesql.

Consider the following scenario:

<code class="language-sql">declare @tablename varchar(50)
set @tablename = 'test'
select * from @tablename</code>

This static query will fail because the table name is dynamically assigned.

Dynamic query example:

The following script demonstrates how to use dynamic queries to compare data in two databases:

<code class="language-sql">declare @schema sysname;
declare @table sysname;
declare @query nvarchar(max);

set @schema = 'dbo'
set @table = 'ACTY'

set @query = '
SELECT * FROM [DB_ONE].' + QUOTENAME(@schema) + '.' + QUOTENAME(@table) + '
EXCEPT
SELECT * FROM [DB_TWO].' + QUOTENAME(@schema) + '.' + QUOTENAME(@table);

EXEC sp_executesql @query</code>

In this example, the table names and schema are dynamic, allowing easy comparison of data in different databases.

Note:

Dynamic queries require careful consideration and maintenance. It is recommended to consult resources such as "The Curse and Blessing of Dynamic SQL" to learn about best practices.

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