Interview Questions
PHP offers multiple ways to connect to MySQL: 1) MySQLi (object-oriented and procedural), 2) PDO (PHP Data Objects), and 3) mysql_* functions (deprecated, shouldn't be used). PDO is preferred as it supports multiple database types and offers better security features.
PDO (PHP Data Objects) is a database abstraction layer providing consistent methods to work with multiple databases. Advantages include: database portability, prepared statements support, consistent error handling, and support for transactions. It provides a secure and flexible approach to database operations.
Prepared statements separate SQL logic from data by using placeholders for values. The database treats these values as data rather than part of the SQL command, preventing injection attacks. Values are automatically escaped, and the query structure remains constant, improving security and performance.
A transaction is a sequence of operations that must be executed as a single unit. In PHP, transactions are implemented using beginTransaction(), commit(), and rollback() methods. If any operation fails, rollback() ensures all operations are undone, maintaining data integrity.
mysql_real_escape_string() escapes special characters in strings, but is deprecated and can be bypassed. Prepared statements are more secure as they separate SQL from data, handle different data types automatically, and are more efficient due to query preparation and caching.
PDO offers several fetch modes: FETCH_ASSOC (returns associative array), FETCH_NUM (returns numeric array), FETCH_BOTH (returns both), FETCH_OBJ (returns object), FETCH_CLASS (returns instance of specified class), and FETCH_LAZY (allows property access of all three).
Database connection errors can be handled using try-catch blocks with PDOException for PDO, or mysqli_connect_errno() and mysqli_connect_error() for MySQLi. Good practice includes logging errors, displaying user-friendly messages, and implementing connection retry logic.
LIMIT controls the maximum number of records returned, while OFFSET specifies where to start returning records. These are commonly used for pagination. In PHP, they're often used with prepared statements: 'SELECT * FROM table LIMIT ? OFFSET ?'
Database connection pooling can be implemented using persistent connections (PDO::ATTR_PERSISTENT), connection management libraries like PHP-CP, or external connection poolers like PgBouncer. This helps reduce connection overhead and improve application performance.
Database migrations are version control for databases, tracking changes to database schema. They ensure consistent database structure across different environments, enable rollback of changes, and facilitate team collaboration. Tools like Phinx or Laravel Migrations help manage this process.
Database seeders are scripts that populate a database with initial or test data. They're useful for development environments, testing, and providing default data. Seeders help ensure consistent data across different environments and make testing more reliable.
Query optimization techniques include: using indexes properly, selecting only needed columns, using EXPLAIN to analyze queries, implementing caching, using prepared statements, limiting result sets, optimizing JOIN operations, and avoiding N+1 query problems.
N+1 query problem occurs when code executes N additional queries to fetch related data for N results. It can be solved using eager loading (JOIN queries), implementing proper indexing, using subqueries, or utilizing ORM features like with() in Laravel.
Database sharding involves distributing data across multiple databases. Implementation includes: defining sharding key, creating routing logic, managing cross-shard queries, and handling transactions. PHP frameworks or custom implementations can manage connection routing and data distribution.
Indexes are data structures that improve the speed of data retrieval operations. They should be used on columns frequently used in WHERE clauses, JOIN conditions, and ORDER BY statements. However, they add overhead to write operations and consume storage space.
Deadlocks can be handled by: implementing retry logic with try-catch blocks, using proper transaction isolation levels, ordering operations consistently, minimizing transaction duration, implementing timeouts, and logging deadlock incidents for analysis.
Normalization is organizing database tables to minimize redundancy and dependency. It involves dividing large tables into smaller ones and defining relationships. This improves data integrity, reduces anomalies, and makes the database more efficient and maintainable.
Database caching can be implemented using: memory caching (Redis, Memcached), query caching, full-page caching, or ORM caching. Proper cache invalidation strategies, TTL settings, and cache tags help maintain data consistency while improving performance.
Triggers are special procedures that automatically execute when certain database events occur (INSERT, UPDATE, DELETE). In PHP, triggers are defined at database level but can be created and managed through PHP code. They help maintain data integrity and automate actions.
Database replication involves configuring master-slave setup, implementing read-write splitting in code, handling replication lag, and managing failover. PHP applications can use different connections for read/write operations and implement logic to handle replication issues.
Stored procedures are pre-compiled SQL statements stored in the database. In PHP, they're called using CALL statement with PDO or MySQLi. They can improve performance, reduce network traffic, and encapsulate business logic at database level.
Large datasets can be handled using: cursor-based pagination, chunked processing, generator functions, memory-efficient algorithms, background job processing, and streaming results. Proper indexing and query optimization are also crucial.
Lazy loading delays database connection initialization until it's actually needed. This saves resources by not establishing connections unnecessarily. It's implemented by wrapping connection logic in methods that are called only when database access is required.
Database versioning can be implemented using migration tools, version control for schema files, semantic versioning for database changes, and proper documentation. This ensures consistent database state across environments and facilitates deployment.
Views are virtual tables based on result sets of SQL statements. In PHP, they're queried like regular tables but provide benefits like data abstraction, security through limited access, and simplified complex queries. They help maintain clean application architecture.
Database backup can be implemented using PHP functions to execute system commands, dedicated backup libraries, or framework tools. Important aspects include scheduling backups, compression, secure storage, verification, and testing recovery procedures.
Connection pooling maintains a cache of database connections for reuse, reducing the overhead of creating new connections. Benefits include improved performance, better resource utilization, and connection management. It's especially useful in high-traffic applications.
Multi-tenancy can be implemented through separate databases, shared database with separate schemas, or shared tables with tenant IDs. PHP code manages tenant identification, data isolation, and connection routing. Proper security measures ensure data separation.
Database events are notifications of database changes that can trigger PHP code execution. They can be handled using event listeners, message queues, or polling mechanisms. This enables real-time updates and maintaining data consistency across systems.
Connection retry logic involves implementing exponential backoff, maximum retry attempts, proper error handling, and logging. It helps handle temporary connection issues and improves application reliability. Implementation typically uses try-catch blocks with sleep intervals.
PHP offers multiple ways to connect to MySQL: 1) MySQLi (object-oriented and procedural), 2) PDO (PHP Data Objects), and 3) mysql_* functions (deprecated, shouldn't be used). PDO is preferred as it supports multiple database types and offers better security features.
PDO (PHP Data Objects) is a database abstraction layer providing consistent methods to work with multiple databases. Advantages include: database portability, prepared statements support, consistent error handling, and support for transactions. It provides a secure and flexible approach to database operations.
Prepared statements separate SQL logic from data by using placeholders for values. The database treats these values as data rather than part of the SQL command, preventing injection attacks. Values are automatically escaped, and the query structure remains constant, improving security and performance.
A transaction is a sequence of operations that must be executed as a single unit. In PHP, transactions are implemented using beginTransaction(), commit(), and rollback() methods. If any operation fails, rollback() ensures all operations are undone, maintaining data integrity.
mysql_real_escape_string() escapes special characters in strings, but is deprecated and can be bypassed. Prepared statements are more secure as they separate SQL from data, handle different data types automatically, and are more efficient due to query preparation and caching.
PDO offers several fetch modes: FETCH_ASSOC (returns associative array), FETCH_NUM (returns numeric array), FETCH_BOTH (returns both), FETCH_OBJ (returns object), FETCH_CLASS (returns instance of specified class), and FETCH_LAZY (allows property access of all three).
Database connection errors can be handled using try-catch blocks with PDOException for PDO, or mysqli_connect_errno() and mysqli_connect_error() for MySQLi. Good practice includes logging errors, displaying user-friendly messages, and implementing connection retry logic.
LIMIT controls the maximum number of records returned, while OFFSET specifies where to start returning records. These are commonly used for pagination. In PHP, they're often used with prepared statements: 'SELECT * FROM table LIMIT ? OFFSET ?'
Database connection pooling can be implemented using persistent connections (PDO::ATTR_PERSISTENT), connection management libraries like PHP-CP, or external connection poolers like PgBouncer. This helps reduce connection overhead and improve application performance.
Database migrations are version control for databases, tracking changes to database schema. They ensure consistent database structure across different environments, enable rollback of changes, and facilitate team collaboration. Tools like Phinx or Laravel Migrations help manage this process.
Database seeders are scripts that populate a database with initial or test data. They're useful for development environments, testing, and providing default data. Seeders help ensure consistent data across different environments and make testing more reliable.
Query optimization techniques include: using indexes properly, selecting only needed columns, using EXPLAIN to analyze queries, implementing caching, using prepared statements, limiting result sets, optimizing JOIN operations, and avoiding N+1 query problems.
N+1 query problem occurs when code executes N additional queries to fetch related data for N results. It can be solved using eager loading (JOIN queries), implementing proper indexing, using subqueries, or utilizing ORM features like with() in Laravel.
Database sharding involves distributing data across multiple databases. Implementation includes: defining sharding key, creating routing logic, managing cross-shard queries, and handling transactions. PHP frameworks or custom implementations can manage connection routing and data distribution.
Indexes are data structures that improve the speed of data retrieval operations. They should be used on columns frequently used in WHERE clauses, JOIN conditions, and ORDER BY statements. However, they add overhead to write operations and consume storage space.
Deadlocks can be handled by: implementing retry logic with try-catch blocks, using proper transaction isolation levels, ordering operations consistently, minimizing transaction duration, implementing timeouts, and logging deadlock incidents for analysis.
Normalization is organizing database tables to minimize redundancy and dependency. It involves dividing large tables into smaller ones and defining relationships. This improves data integrity, reduces anomalies, and makes the database more efficient and maintainable.
Database caching can be implemented using: memory caching (Redis, Memcached), query caching, full-page caching, or ORM caching. Proper cache invalidation strategies, TTL settings, and cache tags help maintain data consistency while improving performance.
Triggers are special procedures that automatically execute when certain database events occur (INSERT, UPDATE, DELETE). In PHP, triggers are defined at database level but can be created and managed through PHP code. They help maintain data integrity and automate actions.
Database replication involves configuring master-slave setup, implementing read-write splitting in code, handling replication lag, and managing failover. PHP applications can use different connections for read/write operations and implement logic to handle replication issues.
Stored procedures are pre-compiled SQL statements stored in the database. In PHP, they're called using CALL statement with PDO or MySQLi. They can improve performance, reduce network traffic, and encapsulate business logic at database level.
Large datasets can be handled using: cursor-based pagination, chunked processing, generator functions, memory-efficient algorithms, background job processing, and streaming results. Proper indexing and query optimization are also crucial.
Lazy loading delays database connection initialization until it's actually needed. This saves resources by not establishing connections unnecessarily. It's implemented by wrapping connection logic in methods that are called only when database access is required.
Database versioning can be implemented using migration tools, version control for schema files, semantic versioning for database changes, and proper documentation. This ensures consistent database state across environments and facilitates deployment.
Views are virtual tables based on result sets of SQL statements. In PHP, they're queried like regular tables but provide benefits like data abstraction, security through limited access, and simplified complex queries. They help maintain clean application architecture.
Database backup can be implemented using PHP functions to execute system commands, dedicated backup libraries, or framework tools. Important aspects include scheduling backups, compression, secure storage, verification, and testing recovery procedures.
Connection pooling maintains a cache of database connections for reuse, reducing the overhead of creating new connections. Benefits include improved performance, better resource utilization, and connection management. It's especially useful in high-traffic applications.
Multi-tenancy can be implemented through separate databases, shared database with separate schemas, or shared tables with tenant IDs. PHP code manages tenant identification, data isolation, and connection routing. Proper security measures ensure data separation.
Database events are notifications of database changes that can trigger PHP code execution. They can be handled using event listeners, message queues, or polling mechanisms. This enables real-time updates and maintaining data consistency across systems.
Connection retry logic involves implementing exponential backoff, maximum retry attempts, proper error handling, and logging. It helps handle temporary connection issues and improves application reliability. Implementation typically uses try-catch blocks with sleep intervals.