Crafting Dynamic SQL in SQL Server to Drop Tables by Prefix

• SQL statements are typically static, meaning they are written entirely before being executed.
• Dynamic SQL, on the other hand, allows you to construct SQL statements at runtime using programming elements. This is useful when you need to build the SQL statement based on external factors, like user input or variable values.

Dropping Tables with a Prefix:

Here's how to achieve this using dynamic SQL in SQL Server:

• Declare a variable:

• Construct the SQL statement:

  • A SELECT statement retrieves table names from the sys.tables system view, filtering them based on the LIKE operator with a wildcard character (%). This ensures names that start with the specified prefix are selected.
  • Within the SELECT statement, the QUOTENAME function is used around the table name (name) to ensure proper handling of special characters or spaces.
  • The retrieved table names are then concatenated to the @sql variable, prefixing each with 'DROP TABLE'.

• Execute the dynamic SQL:

Benefits of using dynamic SQL:

• Flexibility: You can easily modify the prefix by changing the value passed to the LIKE operator.
• Reusability: The core logic can be reused for dropping tables based on different prefixes.

Important Considerations:

• Review the generated SQL: It's recommended to print the @sql variable content before executing it with sp_executesql. This allows you to review the exact statements that will be run and verify they align with your expectations.
• Data Loss: Dropping tables is an irreversible operation that leads to data loss. Ensure you have proper backups before executing this script.
• Foreign Key Constraints: If the tables you're dropping have foreign key relationships with other tables, you might need to address those constraints beforehand to avoid errors during the drop operation.

DECLARE @sql NVARCHAR(MAX) = N'';

-- Replace 'my_prefix' with the actual prefix you want to target
SELECT @sql += 'DROP TABLE ' + QUOTENAME(name) + ';'
FROM sys.tables
WHERE name LIKE 'my_prefix%';

-- Print the generated SQL for review (optional but recommended)
PRINT @sql;

-- Execute the dynamic SQL (uncomment if you're sure about the generated statements)
-- EXEC sp_executesql @sql;

Explanation:

1. The SELECT statement constructs the dynamic SQL. It retrieves table names from sys.tables where the name LIKEs 'my_prefix%'. The QUOTENAME function ensures proper handling of table names.
2. Each retrieved table name is concatenated to @sql prefixed with 'DROP TABLE'.
3. The PRINT statement (optional) displays the generated SQL for review before execution. This allows you to verify if the tables you intend to drop are correctly identified.
4. The line with sp_executesql is commented out by default. Uncomment it only if you're sure about the generated SQL statements and want to proceed with dropping the tables.

Important:

• Replace 'my_prefix' with the actual prefix you want to target.
• Remember to comment back the sp_executesql line only after reviewing the printed SQL statement.

This method iterates through a cursor containing the list of tables with the desired prefix and drops them one by one.

DECLARE @tableName SYSNAME;

-- Cursor to iterate through table names
DECLARE table_cursor CURSOR FOR
SELECT name FROM sys.tables WHERE name LIKE 'my_prefix%';

OPEN table_cursor;

FETCH NEXT FROM table_cursor INTO @tableName;

WHILE @@FETCH_STATUS = 0
BEGIN
  -- Drop the current table
  DROP TABLE @tableName;

  FETCH NEXT FROM table_cursor INTO @tableName;
END;

CLOSE table_cursor;
DEALLOCATE table_cursor;

1. A cursor named table_cursor is declared to iterate through table names.
2. The SELECT statement populates the cursor with names from sys.tables that match the prefix.
3. The OPEN table_cursor statement opens the cursor for iteration.
4. A WHILE loop iterates as long as there are more rows (@@FETCH_STATUS = 0).
5. Inside the loop, FETCH NEXT FROM table_cursor retrieves the next table name into the @tableName variable.
6. The DROP TABLE @tableName statement drops the current table.
7. The loop continues fetching and dropping tables until all rows are processed.
8. Finally, CLOSE table_cursor and DEALLOCATE table_cursor are used to close and release resources associated with the cursor.

System Stored Procedures:

SQL Server provides system stored procedures like sp_msforeachtable that can be used to iterate through all tables and perform actions based on a condition. However, these procedures might not be as flexible for specific prefix targeting.

Considerations:

• Cursors can be less performant compared to dynamic SQL for large numbers of tables.
• System stored procedures might require additional logic to filter based on the prefix.