Understanding SQL Transactions: Implicit vs Explicit and the nuances of transaction management in SQL Server

In SQL, transactions are essential for ensuring data integrity, consistency, and reliability during database operations. They provide a way to group multiple actions into a single unit of work, ensuring that either all changes are committed or none are applied in case of failure (Atomic).

While transactions can be managed explicitly using specific commands (explicit transactions), SQL Server also handles transactions automatically (implicit transactions) for simpler operations.

Including BEGIN TRANSACTION, COMMIT, and ROLLBACK in your SQL code is a good practice when the operation involves multiple steps or when there is a need to ensure atomicity (all steps succeed or none are applied). If the UPDATE operation is the only operation you are performing and does not depend on other complex logic, you do not need to manually include explicit transactions because SQL Server, by default, wraps single UPDATE statements in an implicit transaction.

Transaction boundaries
Transaction boundaries define the start and end points of a database transaction. They encapsulate a series of operations that must be treated as a single, atomic unit. Within these boundaries, either all operations succeed (commit) or none are applied (rollback) to ensure consistency and integrity of the data.

Components of Transaction Boundaries

1. Start of a Transaction:
   • Marked by BEGIN TRANSACTION in SQL.
   • Indicates the beginning of a logical group of operations.
2. Commit (Successful End):
   • Marked by COMMIT TRANSACTION.
   • Saves all the changes made during the transaction to the database
3. Rollback (Unsuccessful End):
   • Marked by ROLLBACK TRANSACTION.
   • Undoes all changes made during the transaction.
   • Used when an error occurs or a condition fails within the transaction boundary.
4. Implicit and Explicit Transactions:
   • Implicit Transactions: SQL Server wraps individual DML statements like UPDATE, INSERT, or DELETE in a transaction automatically.
   • Explicit Transactions: Developers explicitly define the transaction boundaries using BEGIN TRANSACTION, COMMIT, and ROLLBACK.

The Two Transaction Types
Implicit

• Implicit transactions are automatically managed by SQL Server and automatically start a transaction whenever a data-modifying statement such as INSERT, UPDATE, or DELETE is executed. This is helpful in situations where you want SQL Server to handle the transaction for you without having to manually manage it. This is ideal for simple scripts where you don't need full control over transaction boundaries.
• Implicit transactions are tied to the connection context. If the connection is closed or disconnected, then the transaction will be rolled back automatically.

Explicit

• Explicit transactions are not automatically managed by SQL Server and require you to define (and manage) the transaction boundaries of your SQL actions using the BEGIN TRANSACTION, COMMIT TRANSACTION, and ROLLBACK TRANSACTION commands. This allows more control over when the transaction begins and ends.
• Explicit transactions can span multiple operations and even across multiple batches or procedures as long as the same connection remains open.

Atomicity, Consistency, Isolation, Durability (ACID) Properties (a lot of highfalutin words)

Atomicity:
Transactions are atomic. What this means is that a transaction (UPDATE, INSERT, or DELETE) will either complete entirely or not at all (basically a ROLLBACK).

Consistency:
Transactions move the database from one consistent state to another.
Basically, what this means is that transactions ensure that the database remains in a valid state before and after a transaction. It means all defined rules (constraints, relationships, etc.) are adhered to, preventing data corruption or invalid states. If a transaction violates any constraint, the database reverts to its previous consistent state (basically a ROLLBACK).

Isolation:
This ensures that concurrent transactions (running at the same time) do not interfere with each other which maintains the integrity of each transactions operations. Basically, it allows transactions to execute as if they were running sequentially, preventing issues like dirty reads, non-repeatable reads, or phantom reads, depending on the isolation level.

Durability:
Once a transaction is committed, the changes are permanent, even in the event of a system failure.

When to Include Transactions:

• If multiple statements need to succeed or fail as a group (ACID).
• If the operation involves dependencies or cascading changes across multiple tables. An example of something like this would be deleting a primary parent (ex. customer) record, which requires cascading deletions of all related child records (ex. orders and payments) to maintain referential integrity across the tables.
• If future enhancements might make the procedure more complex.

Performance Considerations

• Implicit transactions could unintentionally incur additional overhead due to SQL Server managing the transaction boundaries automatically.
• Explicit transactions can sometimes be more efficient because you have complete control over when transactions start and end (the boundaries).
• In a high-performance environment you should use explicit transactions to minimize unnecessary transaction overhead and ensure that transactions only last as long as necessary.

Some Randomly Related Stuff
Error Handling and Explicit Transactions
Always use error handling (TRY...CATCH), keep your transactions as short as you can, and only include statements that need to be part of the same transaction.

TRY...CATCH blocks are used to catch errors and perform a ROLLBACK if something goes wrong.

Error Handling and Implicit Transactions
Implicit transactions are more prone to unintended commits if error handling is not properly implemented.

What does this mean? Implicit transactions are automatically committed (by SQL Server) after each individual statement completes successfully (INSERT, UPDATE, DELETE, etc.) without explicit control over the transaction boundaries. If an error occurs mid-operation then the already completed statements cannot be rolled back. This could leave your database in an inconsistent state. Because of this, using error handling and explicit transactions (BEGIN, COMMIT, ROLLBACK) provide you with greater control and ensure atomicity and consistency.

A very simplistic TSQL example of Explicit Transaction
This example code snippet is for illustrative purposes only and does not necessarily represent best coding practices.

declare @theid int = 5

begin try
  begin transaction MyNamedTransaction

    -- insert into "YourSQLTable"
    insert into [dbo].[YourSQLTable] (Column1, Column2, Column3, Column4, Column5)
    values (@theid, 'some text value', 2, 3, getdate())

    -- update "AnotherSQLTable"
    update [dbo].[AnotherSQLTable]
    set [somecolumn] = 1,
    [anothercolumn] = (select [columnname] from [dbo].[YourSQLTable] where [pid] = @theid)
    where [cid] = @theid

  commit transaction MyNamedTransaction
end try
begin catch
  if @@trancount > 0
  begin  
    rollback transaction MyNamedTransaction
  end    
  declare @message nvarchar(2048) = error_message()
  insert into [dbo].[MyErrorLogTable] (ErrorMessage, ErrorDate) values (@message, getdate())
  throw
end catch

Conclusion
Choosing between implicit and explicit transactions depends on the complexity of your database operations.

Implicit transactions are ideal for simple, single-step actions that require atomicity without the need for explicit transaction management.

As operations become more complex and involve multiple actions or tables, explicit transactions provide more control and flexibility.

By understanding implicit and explicit transactions (and when to use them or not), you can ensure your SQL code is both efficient and reliable, maintaining data integrity and minimizing errors.