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Mastering Temporary Tables in SQL Server: Select Into vs Create Table

Creating Temporary Tables in SQL Server

When working within a database, it may be necessary to create temporary tables to store data that will only be used for a short period of time. Temporary tables are useful for handling large amounts of data, creating intermediary results during a complex process, or providing a workspace for experimentation without disrupting permanent data.

SQL Server offers two main methods for creating temporary tables: the Select Into approach and the Create Table approach. In this article, we will explore the steps for creating temporary tables in SQL Server and provide examples of how each approach is used.

Select Into Approach

The Select Into approach is a simple and fast way to create a temporary table in SQL Server. This method involves selecting data from an existing table and inserting it into a newly created temporary table.

The syntax for the Select Into approach is as follows:

SELECT * INTO #temp_table_name
FROM existing_table
WHERE condition;

The Select Into approach creates a temporary table with the same columns and data types as the existing table. You can specify which columns to include in the temporary table by selecting only those columns from the existing table.

Additionally, you can add conditions to limit the data that is copied into the temporary table. This approach is particularly useful when working with large datasets, as it allows for a quick and efficient method of creating a temporary table with only the data needed.

Example: Creating a Temporary Table from an Existing Table

Let’s say we have a table named “sales_data” with columns for “sales_id,” “customer_id,” “month,” and “sales_amt.” We want to create a temporary table that will store only the sales data for the month of January. Here’s how we would use the Select Into approach to accomplish this:

SELECT sales_id, customer_id, sales_amt INTO #jan_sales
FROM sales_data
WHERE month = 'January';

This statement creates a temporary table named “#jan_sales” with the same columns and data types as “sales_data,” but only includes the data where the “month” column equals “January.” We now have a temporary table that contains only the data we need for our current task.

Dropping the Temporary Table

Temporary tables are not permanent and will be automatically dropped when the session that created them is closed. However, it is good practice to explicitly drop the temporary table once it is no longer needed.

To drop a temporary table, use the following statement:

DROP TABLE #temp_table_name;

This statement will remove the temporary table from the database. It is important to note that dropping a temporary table will also remove all data stored in that table.

Create Table Approach

The Create Table approach involves explicitly creating a temporary table with a specified set of columns and data types. This method is useful when you need to create a temporary table that has a specific structure that differs from the existing table.

The Create Table approach requires two steps: creating the table and inserting data into it. The syntax for the Create Table approach is as follows:

CREATE TABLE #temp_table_name (
    column1 datatype,
    column2 datatype,
    column3 datatype,
    ...
);

INSERT INTO #temp_table_name (
    column1,
    column2,
    column3,
    ... )
SELECT column1,
       column2,
       column3,
       ...
FROM existing_table
WHERE condition;

The first part of this statement creates a temporary table with the specified columns and data types. The second part inserts data into the temporary table by selecting columns from an existing table and inserting them into the temporary table.

Like the Select Into approach, you can specify which columns to include in the temporary table and add conditions to limit the data inserted. Example: Creating a Temporary Table Using the

Create Table Approach

Let’s say we want to create a temporary table that stores customer data, including “customer_id,” “first_name,” “last_name,” and “email.” However, we only want to select customers who have made a purchase in the last 30 days.

Here’s how we would use the Create Table approach to accomplish this:

CREATE TABLE #recent_customers (
    customer_id INT,
    first_name VARCHAR(50),
    last_name VARCHAR(50),
    email VARCHAR(255)
);

INSERT INTO #recent_customers (
    customer_id,
    first_name,
    last_name,
    email
)
SELECT customer_id,
       first_name,
       last_name,
       email
FROM customer_table
WHERE purchase_date >= DATEADD(day, -30, GETDATE());

This statement creates a temporary table named “#recent_customers” with columns for the customer ID, first name, last name, and email. It then inserts customer data from the “customer_table” where the purchase date is within the last 30 days into the temporary table.

We now have a temporary table that contains only the relevant customer data for our current task.

Dropping the Temporary Table

As with the Select Into approach, temporary tables created using the Create Table approach are not permanent and will be automatically dropped when the session that created them is closed. However, it is good practice to explicitly drop the temporary table once it is no longer needed.

To drop a temporary table, use the following statement:

DROP TABLE #temp_table_name;

This statement will remove the temporary table from the database. It is important to note that dropping a temporary table will also remove all data stored in that table.

3) Creating Temporary Table Using

Create Table Approach

In addition to the Select Into approach, SQL Server also offers the Create Table approach for creating temporary tables. The Create Table approach involves explicitly creating a temporary table with a specified set of columns and data types.

This method is useful when you need to create a temporary table that has a specific structure that differs from the existing table.

Recreating the Temporary Table

While temporary tables are automatically dropped when the session that created them is closed, there may be instances where you need to recreate a temporary table within the same session. To recreate a temporary table using the Create Table approach, first drop the existing temporary table, then recreate it using the same CREATE TABLE statement.

Finally, insert data into the new temporary table using the INSERT INTO statement and the SELECT statement to define the data to be inserted. Here is an example:

DROP TABLE #temp_table_name;

CREATE TABLE #temp_table_name (
    column1 datatype,
    column2 datatype,
    column3 datatype,
    ...
);

INSERT INTO #temp_table_name (
    column1,
    column2,
    column3,
    ... )
SELECT column1,
       column2,
       column3,
       ...
FROM existing_table
WHERE condition;

This statement drops the existing temporary table, creates a new temporary table with the same structure using the CREATE TABLE statement, and then inserts data into the new temporary table using the INSERT INTO statement and the SELECT statement.

Dropping the Temporary Table

As with the Select Into approach, it is important to explicitly drop the temporary table once it is no longer needed to avoid cluttering the database and wasting resources. To drop a temporary table created using the Create Table approach, use the following statement:

DROP TABLE #temp_table_name;

This statement will remove the temporary table from the database.

It is important to note that dropping a temporary table will also remove all data stored in that table.

4) Creating Global Temporary Table

In SQL Server, a global temporary table is a temporary table that is visible to all sessions. Global temporary tables are useful when multiple users need to share data, but you do not want to create permanent tables in the database.

Double Hash Symbol for Global Temporary Table

To distinguish a global temporary table from a local temporary table, you must use a double hash symbol (##) as a prefix for the table name. The syntax for creating a global temporary table is similar to the Create Table approach for local temporary tables, with the addition of the double hash symbol (##) prefix:

CREATE TABLE ##global_temp_table_name (
    column1 datatype,
    column2 datatype,
    column3 datatype,
    ...
);

Example: Creating a Global Temporary Table

Let’s say we have a database that is being used by multiple users, and we need to store a list of employees that can be accessed by all users without creating a permanent table. We can use a global temporary table to accomplish this:

CREATE TABLE ##employee_list (
    employee_id INT,
    first_name VARCHAR(50),
    last_name VARCHAR(50),
    title VARCHAR(255)
);

This statement creates a global temporary table named “##employee_list” with columns for the employee ID, first name, last name, and title.

The global temporary table is now accessible to all sessions.

Dropping the Global Temporary Table

To drop a global temporary table, use the following statement:

DROP TABLE ##global_temp_table_name;

This statement will remove the global temporary table from the database. It is important to note that dropping a global temporary table will also remove all data stored in that table.

Global temporary tables are automatically dropped when the session that created them is closed, but it is good practice to explicitly drop the table once it is no longer needed.

Conclusion

In conclusion, creating temporary tables in SQL Server can be useful for handling large amounts of data, storing intermediary results during a complex process, or providing a workspace for experimentation without disrupting permanent data. SQL Server offers two main methods for creating temporary tables: the Select Into approach and the Create Table approach.

Additionally, there are global temporary tables that can be shared across sessions using double hash symbol (##) as a prefix for the table name. Regardless of the method used, it is important to explicitly drop the temporary table once it is no longer needed to avoid cluttering the database and wasting resources.

In conclusion, temporary tables are useful for handling large amounts of data, storing intermediary results, and providing a workspace for experimentation without disrupting permanent data. There are two main methods for creating temporary tables in SQL Server: the Select Into approach and the Create Table approach.

It is important to explicitly drop the temporary table once it is no longer needed to avoid cluttering the database and wasting resources. Additionally, global temporary tables can be shared across sessions using a double hash symbol (##) as a prefix for the table name.

Leveraging these temporary table techniques can improve the efficiency of your database operations by providing fast and flexible storage for temporary data.

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