Temporary Tables vs. Table Variables: Choosing the Right Tool for the Job (SQL Server)

• Temporary Tables (#temp): Created within the tempdb database and exist for the entire session (unless explicitly dropped). They are visible to all connections within that session.
• Table Variables (@table): Defined like other variables and exist only within the current batch of Transact-SQL statements. They are accessible only by the current connection.

Storage and Performance:

• Temporary Tables: Primarily reside on disk in tempdb. Can leverage indexes for faster querying, especially with larger datasets.
• Table Variables: Ideally stored in memory for quicker access. If the data size spills over available memory, they might be written to tempdb. Generally faster for small datasets due to in-memory storage.

Other Considerations:

• Schema Changes: Temporary tables allow modifications like adding columns or constraints. Table variables do not.
• Transactions and Locking: Temporary tables can participate in transactions and locking mechanisms. Table variables cannot.

Choosing Between Them:

• Temporary tables: Use for larger datasets, complex queries requiring indexes, or scenarios involving transactions and locking.
• Table variables: Ideal for small datasets, simple queries where speed is crucial, and situations where you only need the data within a single batch.

-- Create a temporary table
CREATE TABLE #Products (
  ProductID int PRIMARY KEY,
  ProductName nvarchar(50) NOT NULL
);

-- Insert data into the temporary table
INSERT INTO #Products (ProductID, ProductName)
VALUES (1, 'Widget');

-- Select data from the temporary table
SELECT * FROM #Products;

-- Drop the temporary table
DROP TABLE #Products;

Table Variable:

-- Declare a table variable
DECLARE @Customers TABLE (
  CustomerID int PRIMARY KEY,
  CustomerName nvarchar(50) NOT NULL
);

-- Insert data into the table variable
INSERT INTO @Customers (CustomerID, CustomerName)
VALUES (101, 'Alice');

-- Select data from the table variable
SELECT * FROM @Customers;

• Use Case: Suitable for complex result sets needing intermediate processing steps within a single query.
• Benefits: Avoids creating temporary objects, improves readability, and can be reused within the same query.

Example:

WITH OrderedCustomers AS (
  SELECT CustomerID, CustomerName
  FROM Customers
  ORDER BY CustomerName
)
SELECT * FROM OrderedCustomers;

Subqueries:

• Use Case: Effective for simpler scenarios where you need to filter or transform data based on another result set within the same query.
• Benefits: More concise for straightforward operations compared to creating temporary structures.

SELECT * FROM Orders
WHERE CustomerID IN (
  SELECT CustomerID FROM Customers WHERE Country = 'USA'
);

Memory-Optimized Tables (SQL Server 2014 and above):

• Use Case: Ideal for frequently accessed, performance-critical datasets that can entirely reside in memory.
• Benefits: Significantly faster access compared to temporary tables stored on disk.

CREATE TABLE dbo.InMemoryProducts (
  ProductID int PRIMARY KEY,
  ProductName nvarchar(50) NOT NULL
) WITH (MEMORY_OPTIMIZED = ON);

-- Similar usage pattern as temporary tables for data manipulation