Q: How do I safely run UPDATE or DELETE?

Always include WHERE clause. Without WHERE, UPDATE modifies all rows, DELETE removes all rows. Safety steps: 1) Run SELECT first to verify target rows. 2) Add LIMIT to restrict affected rows. 3) Use transactions (BEGIN, COMMIT, ROLLBACK). 4) Backup before major changes. Test queries in development first.

Q: What is SQL injection and how to prevent it?

SQL injection occurs when user input contains malicious SQL. Example: input '1; DELETE FROM users' in WHERE id = input. Prevention: use prepared statements / parameterized queries, never concatenate user input into SQL, use ORM libraries (Prisma, Sequelize), validate input before using in queries.

Q: How do I join multiple tables in SQL?

JOIN combines data from multiple tables. Types: INNER JOIN (matching rows only), LEFT JOIN (all left rows + matching right), RIGHT JOIN (all right rows + matching left), FULL JOIN (all rows from both). Syntax: SELECT a.name, b.order FROM users a JOIN orders b ON a.id = b.user_id. Use aliases for readability.

Question 1

What are the basic SQL query types?

Accepted Answer

Four basic SQL operations: SELECT (retrieve data), INSERT (add new rows), UPDATE (modify existing rows), DELETE (remove rows). Known as CRUD: Create (INSERT), Read (SELECT), Update (UPDATE), Delete (DELETE). Most database operations use these four query types.

Question 2

How do I write a SELECT query with conditions?

Accepted Answer

SELECT columns FROM table WHERE condition. Example: SELECT name, email FROM users WHERE status = 'active' AND age > 18 ORDER BY created_at DESC LIMIT 10. WHERE filters rows, ORDER BY sorts results, LIMIT restricts count. Use AND/OR for multiple conditions. Operators: =, !=, >, <, LIKE, IN, BETWEEN.

Question 3

How do I safely run UPDATE or DELETE?

Accepted Answer

Always include WHERE clause. Without WHERE, UPDATE modifies all rows, DELETE removes all rows. Safety steps: 1) Run SELECT first to verify target rows. 2) Add LIMIT to restrict affected rows. 3) Use transactions (BEGIN, COMMIT, ROLLBACK). 4) Backup before major changes. Test queries in development first.

Question 4

What is SQL injection and how to prevent it?

Accepted Answer

SQL injection occurs when user input contains malicious SQL. Example: input '1; DELETE FROM users' in WHERE id = input. Prevention: use prepared statements / parameterized queries, never concatenate user input into SQL, use ORM libraries (Prisma, Sequelize), validate input before using in queries.

Question 5

How do I join multiple tables in SQL?

Accepted Answer

JOIN combines data from multiple tables. Types: INNER JOIN (matching rows only), LEFT JOIN (all left rows + matching right), RIGHT JOIN (all right rows + matching left), FULL JOIN (all rows from both). Syntax: SELECT a.name, b.order FROM users a JOIN orders b ON a.id = b.user_id. Use aliases for readability.

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