SQL
Constraints
Constraints enforce rules at the column level to maintain data integrity:
| Constraint | Description |
|---|---|
| PRIMARY KEY | Uniquely identifies each record in a table. |
| FOREIGN KEY | Establishes relationships between tables. |
| NOT NULL | Ensures a column cannot have NULL values. |
| UNIQUE | Ensures all values in a column are different. |
| CHECK | Specifies conditions that must be met for data to be valid. |
Transactions
SQL supports transactions to ensure data integrity:
| Transaction | Description |
|---|---|
| COMMIT | Saves changes made during the current transaction. |
| ROLLBACK | Restores the database to its state before the current transaction started. |
Joins
SQL allows combining data from multiple tables using JOIN operations:
| Join | Description |
|---|---|
| INNER JOIN | Retrieves records that have matching values in both tables. |
| LEFT JOIN | Retrieves all records from the left table and matching records from the right table. |
| RIGHT JOIN | Retrieves all records from the right table and matching records from the left table. |
| FULL JOIN | Retrieves all records when there is a match in either left or right table. |
SQL is essential for managing and querying databases efficiently, making it a fundamental skill for database administrators, developers, and data analysts working with relational databases.
General Tips!
- Use Proper Indexing: Index frequently queried columns to improve query performance, but avoid over-indexing as it can slow down write operations.
- Write Efficient Queries: Optimize your queries by selecting only the necessary columns, avoiding SELECT *, and using joins wisely.
- Normalize Your Data: Organize your database into normalized tables to reduce redundancy and improve data integrity.
- Use Transactions: Ensure data consistency and integrity by wrapping multiple related operations in transactions, especially in multi-step processes.
- Understand and Use SQL Functions: Leverage built-in SQL functions for data manipulation and retrieval, such as aggregate functions (SUM, COUNT) and string functions (CONCAT, SUBSTRING).