How do you handle errors in SQL procedures?

You use TRY...CATCH blocks to detect errors, roll back transactions, and optionally log or raise custom errors using RAISERROR or THROW.

What is the benefit of using transactions in error handling?

Transactions ensure that either all steps succeed or none are applied, preventing partial or inconsistent updates in case of failure.

SQL Error Handling in Procedures and Scripts | FullStackPrep.Dev

💡 Concept Name

SQL Error Handling – A set of techniques and constructs like TRY...CATCH, RAISERROR, and transactions used to catch, handle, or log errors gracefully in SQL scripts and stored procedures.

📘 Quick Intro

Error handling ensures your SQL procedures remain consistent and stable during failures like divide-by-zero, constraint violations, or failed inserts. SQL Server uses TRY...CATCH blocks to detect and respond to these errors programmatically.

🧠 Analogy / Short Story

Imagine assembling furniture. If a part is missing, you stop, check the manual, and either skip that step or return the product. SQL error handling is similar—it checks if an operation fails, logs or reports it, and either rolls back or exits gracefully instead of crashing mid-execution.

🔧 Technical Explanation

TRY...CATCH lets you catch runtime errors in SQL Server and handle them in the CATCH block.
Functions like ERROR_MESSAGE(), ERROR_NUMBER(), and ERROR_LINE() retrieve error details.
RAISERROR or THROW lets you raise custom errors with severity and state.
Transactions can be used with error handling to ROLLBACK if anything fails.
Other DBMSs like PostgreSQL use BEGIN ... EXCEPTION WHEN blocks instead.

💻 Real Code Example

BEGIN TRY
    BEGIN TRANSACTION

    -- Intentional divide by zero
    DECLARE @x INT = 1 / 0;

    COMMIT TRANSACTION
END TRY
BEGIN CATCH
    ROLLBACK TRANSACTION;

    PRINT 'Error Number: ' + CAST(ERROR_NUMBER() AS VARCHAR);
    PRINT 'Message: ' + ERROR_MESSAGE();
END CATCH

🎯 Purpose & Use Case

✅ Prevent inconsistent data by rolling back failed transactions.
✅ Log error details to a custom error log table.
✅ Provide user-friendly messages for apps consuming stored procedures.
✅ Ensure mission-critical operations fail gracefully without halting the entire system.

❓ Interview Q&A

Q1: How do you handle runtime errors in SQL Server?
A: Using TRY...CATCH blocks to catch exceptions and take corrective action like ROLLBACK.

Q2: What is the role of ROLLBACK in error handling?
A: It undoes all changes in the current transaction to maintain consistency.

Q3: What function returns the actual error message in SQL Server?
A: ERROR_MESSAGE().

Q4: Can you raise a custom error in SQL Server?
A: Yes, using RAISERROR or THROW.

Q5: What is a common use of error logging?
A: Inserting error info into a log table for auditing/debugging.

Q6: Can a transaction be resumed after a CATCH block?
A: No, once rolled back, a new transaction must be started.

Q7: What happens if you don’t handle errors in SQL?
A: The script halts and may leave the database in an inconsistent state.

Q8: Are error-handling features the same in all RDBMS?
A: No, syntax and functions differ across SQL Server, PostgreSQL, MySQL, etc.

Q9: What’s a good pattern for robust SQL scripts?
A: Combine TRY...CATCH with transactions and logging.

Q10: How can errors be logged automatically?
A: Insert error details into a dedicated error log table in the CATCH block.

📝 MCQs

Q1. What SQL construct catches errors?

IF...ELSE
BEGIN...END
TRY...CATCH
WHILE...BREAK

Q2. Which function returns the SQL error message?

RAISEERROR()
GET_ERROR()
ERROR_MSG()
ERROR_MESSAGE()

Q3. What does ROLLBACK do?

Commits the transaction
Starts a new transaction
Undoes changes in the current transaction
Logs the error

Q4. Which keyword raises a custom error in SQL Server?

THROWEX
RAISE
THROW
RAISERROR

Q5. Why use error logging?

To boost performance
To audit failures
To improve UI
To increase concurrency

Q6. What happens if error is not handled?

Script continues
Script halts and may corrupt data
Database restarts
Transaction pauses

Q7. What clause is used to exit a transaction on error?

BREAK
RETURN
ROLLBACK
RAISE

Q8. What should you check for before COMMIT?

Keys
Errors
Duplicates
Indexes

Q9. How to handle errors in PostgreSQL?

TRY...CATCH
EXCEPTION BLOCK
BEGIN...EXCEPTION
CATCH THROW

Q10. Can you log errors into a table?

No
Yes
Only in triggers
Only with jobs

💡 Bonus Insight

Combine TRY...CATCH blocks with custom error logging tables and RAISERROR levels for a production-grade error recovery strategy. Always wrap critical sections in transactions to prevent partial data persistence.

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SQL Error Handling in Procedures and Scripts