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Lesson 3 Batch queries
Objective Describe batch queries, and compare them to single queries.

Describe SQL Batch Queries

SQL Server 2019, like its predecessors, supports the concept of SQL batch queries, which are fundamental to understanding how SQL Server processes commands. Here's an authoritative overview of SQL batch queries within this context:
  1. Definition: A SQL batch is a group of one or more SQL statements sent to the server at the same time. It's an important concept in SQL Server, as the batch forms the basic unit of work that the server compiles, optimizes, and executes as a unit.
  2. Batch Separation: In SQL Server 2019, batches are separated by the `GO` statement. `GO` is not a SQL command; rather, it's a command recognized by the SQL Server Management Studio (SSMS) and sqlcmd utility, signaling the end of a batch. This allows multiple batches to be written in a single script and executed in sequence.
  3. Compilation and Execution: When a batch is submitted to SQL Server, it undergoes a compilation process. The entire batch is compiled into a single execution plan. This approach can improve performance, as the compilation cost is amortized over the entire batch. The batch is then executed as a single unit.
  4. Error Handling: If an error occurs in one statement within a batch, it can affect the execution of subsequent statements in the same batch. SQL Server 2019 provides tools like `TRY...CATCH` blocks to handle these scenarios, allowing for more granular error control within a batch.
  5. Transactions and Batches: A single batch can contain multiple transactions. However, it's crucial to understand the scope of transactions within batches. A transaction can span multiple batches, but if a batch aborts due to an error, it can roll back the entire transaction, not just the part of the transaction within the batch.
  6. Performance Considerations: Batch queries can significantly enhance performance, especially in network-heavy environments. By reducing the number of round-trips between the application and the database server, batch queries can decrease network latency and increase throughput.
  7. Batch Size Limitations: While large batches can be efficient, they also have limitations. Oversized batches can consume excessive memory and processing resources. SQL Server 2019 enforces certain limits on the size of a batch to maintain system performance and stability.
  8. Use Cases: Batch queries are commonly used for bulk data operations, complex data processing tasks, and situations where multiple SQL statements must be executed atomically.

In summary, SQL batch queries in SQL Server 2019 are a powerful feature that allows for efficient processing of multiple SQL statements. By understanding how to effectively use batches, developers and database administrators can optimize the performance and reliability of their SQL Server applications.

What is a batch?

batch[1] is a group of SQL statements that are executed at one time by SQL Server.
These statements are sent to SQL Server by a program, such as the Query Analyzer. The opposite of a batch query is a single query, containing only one SQL statement. If there are no GO keywords in a set of Transact-SQL statements, they are all considered to be in one batch.
  1. Advantage of Batch Queries: If an error occurs while executing one statement in a batch, SQL Server will not execute the remaining statements. This will help you troubleshoot any problems in your batch files.
  2. When using variables: Each batch is treated as a single unit, and statements in one batch have no knowledge of statements in another batch. Therefore, if you use a variable in a batch, you must declare it within that batch.
  3. When not to batch
    The following CREATE SQL statements can only be executed as single statements:
    These statements cannot be batched because SQL Server does not allow the creation of database objects within a batch.
  4. Creating a batch: The GO statement: All statements that are listed in the Query Analyzer between GO statements are considered to be batches. However, if there are no GO statements, all statements that you type into the Query Analyzer are considered to be a batch. This can be thought of as an implicit GO statement at the very end of all the statements that you send to SQL Server. For example, both statements below are treated as a single batch:
UPDATE Employees
SET FirstName = 'Anthony',
LastName = 'Mann'
WHERE EmployeeID = 101
FROM Employees
WHERE EmployeeID = 101

The following set of statements is treated as two separate batches because they are separated by the GO statement:
USE Timesheets
UPDATE Employees
SET FirstName = 'Anthony',
LastName = 'Mann'
WHERE EmployeeID = 101
FROM Employees
WHERE EmployeeID = 101

In the next lesson, the power of SQL Server system catalogs will be discussed.
[1]Batch: A set of Transact-SQL commands that are sent to the SQL Server engine, all at one time. Batches are separated by GO keywords.