Some of the fundamental SQL commands used to interact with relational databases.

SELECT Statement

The cornerstone of data retrieval in SQL databases.

Key Topics:

• Basic Syntax Structure:

  SELECT column1, column2 FROM table_name WHERE condition;
  

• Column Selection Techniques:

  • Selecting specific columns vs using wildcards (SELECT *)
  • Column aliasing with AS keyword
  • Handling NULL values with COALESCE and IFNULL functions

• Data Filtering with WHERE Clause:

  • Comparison operators (=, <>, <, >, <=, >=)
  • Boolean operators (AND, OR, NOT)
  • Pattern matching with LIKE and wildcards (%, _)
  • Range checking with BETWEEN and IN

• Result Organization:

  • Sorting with ORDER BY (ascending/descending)
  • Grouping with GROUP BY and aggregate functions (COUNT, SUM, AVG, MIN, MAX)
  • Filtering grouped results with HAVING clause
  • Limiting results with LIMIT/OFFSET or TOP/ROWNUM

• Joining Tables:

  • INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN
  • Self-joins and cross joins
  • Using table aliases for cleaner syntax

• Subqueries and Common Table Expressions (CTEs)

  • Using subqueries in SELECT, WHERE, HAVING clauses
  • Using CTEs with the WITH clause for more readable complex queries

INSERT Statement

Adding new records to database tables.

Key Topics:

• Single Row Insertion:

  INSERT INTO table_name (column1, column2) VALUES (value1, value2);
  

• Multiple Row Insertion:

  INSERT INTO table_name (column1, column2) 
  VALUES (value1, value2), (value3, value4), (value5, value6);
  

• Insertion with Subqueries:

  INSERT INTO target_table (column1, column2)
  SELECT column_a, column_b FROM source_table WHERE condition;
  

• Default and Auto-generated Values:

  • Working with identity columns and sequences
  • Using DEFAULT keyword for default values
  • NULL handling during insertion

• Handling Constraint Violations:

  • Managing primary key, unique, foreign key, and check constraints
  • Using IGNORE or ON DUPLICATE KEY options (MySQL)
  • MERGE/UPSERT operations (Oracle, SQL Server)

• Bulk Insert Operations:

  • Optimizing for large data sets
  • Using database-specific bulk loaders
  • Transaction management for batch inserts

UPDATE Statement

Modifying existing records in database tables.

Key Topics:

• Basic Syntax:

  UPDATE table_name SET column1 = value1, column2 = value2 WHERE condition;
  

• Update Techniques:

  • Computing values based on existing data (SET salary = salary * 1.1)
  • Using functions in updates
  • Multi-column updates

• Conditional Updates:

  • WHERE clause best practices
  • Using CASE expressions for complex conditional logic
  • Using subqueries in conditions

• Mass Updates vs. Targeted Updates:

  • Performance considerations
  • Transaction isolation levels
  • Locking implications

• Updating with Joins:

  UPDATE table1 t1
  JOIN table2 t2 ON t1.id = t2.id
  SET t1.column = t2.column
  WHERE condition;
  

• Safe Update Practices:

  • Using transactions for rollback capability
  • Testing updates with SELECT first
  • Using row count verification

DELETE Statement

Removing records from database tables.

Key Topics:

• Basic Syntax:

  DELETE FROM table_name WHERE condition;
  

• Safe Deletion Practices:

  • Using transactions and savepoints
  • Testing with SELECT before executing DELETE
  • Creating deletion scripts with built-in verification

• Targeted vs. Mass Deletion:

  • Performance implications
  • Truncate vs. Delete operations
  • Logging and auditing requirements

• Cascading Deletes:

  • Foreign key constraints with CASCADE options
  • Managing parent-child relationships
  • Orphaned record handling

• Multi-table Deletes:

  DELETE t1 FROM table1 t1
  JOIN table2 t2 ON t1.id = t2.id
  WHERE condition;
  

• Soft Deletes vs. Hard Deletes:

  • Using status flags or timestamps instead of physical deletion
  • Archiving strategies for historical data
  • Recovery mechanisms for accidental deletions

Best Practices Across All Operations

• Transaction Management: Using BEGIN, COMMIT, and ROLLBACK appropriately
• Performance Optimization: Indexing strategies for each operation type
• Security Considerations: Parameterized queries to prevent SQL injection
• Error Handling: Proper error catching and reporting mechanisms
• Database Engine Variations: Syntax differences between major database systems (MySQL, PostgreSQL, SQL Server, Oracle)