SQL UPDATE
Q1. What is the SQL UPDATE command?
The SQL UPDATE command is used to modify existing records in a database table. It works by identifying rows using a WHERE clause and updating specified column values. Internally, the database engine locates rows using indexes or full table scans, applies locks, updates data pages, and records changes in transaction logs. UPDATE is a transactional operation, meaning changes can be committed or rolled back.
Q2. Why is the WHERE clause important in UPDATE statements?
The WHERE clause limits the rows affected by an UPDATE statement. Without it, all rows in the table will be updated, which can cause severe data loss. In interviews, candidates are often warned that forgetting the WHERE clause is a common and costly mistake in production systems.
Q3. How does UPDATE behave in high-concurrency environments?
In high-concurrency environments, UPDATE operations can lead to blocking or deadlocks due to row-level or page-level locks. Proper indexing, short transactions, and batch updates help reduce contention and improve concurrency.
Q4. What are best practices for running UPDATE in production?
Best practices include running a SELECT query first, using transactions, updating data in batches, and monitoring affected rows. These practices reduce risk and improve reliability.
Q5. UPDATE vs MERGE – when should you use each?
UPDATE modifies existing rows, while MERGE combines insert, update, and delete logic in a single statement. MERGE is useful for synchronization scenarios but must be used carefully to avoid unintended data changes.