Comparison Operators are used within the WHERE clause to evaluate data values against specific targets. Common operators include equals (), not equals ( or ), greater than (), and less than (), allowing for precise numerical and text-based filtering.
Logical Operators such as AND, OR, and NOT allow for the construction of complex queries by combining multiple conditions. For example, using AND requires all conditions to be true for a record to be included, while OR requires only one of the conditions to be met.
Pattern Matching can be achieved using the LIKE operator combined with wildcards (such as in many SQL dialects). This allows for searching for partial strings, which is useful when the exact value of a field is unknown or varies.
| Feature | SQL (Database) | Text Files (Flat File) |
|---|---|---|
| Structure | Organized into related tables with defined fields. | Unstructured or simple delimited lines of text. |
| Search Speed | High; uses indexing to find records rapidly. | Low; often requires reading the entire file sequentially. |
| Data Integrity | Enforced through constraints and primary keys. | Minimal; data can easily become inconsistent or duplicated. |
| Concurrency | Supports multiple users accessing data simultaneously. | Usually restricted to one user or process at a time. |
A critical distinction exists between a Field and a Record. A field is a single category of data (a column), whereas a record is a complete set of fields relating to one specific entity (a row).
SQL is a declarative language, which differs from procedural languages like Python or Java. In SQL, you describe the result you want, while in procedural languages, you write the step-by-step logic to achieve that result.
Check Syntax Order: Always ensure the query follows the logical order of SELECT → FROM → WHERE. Reversing these or omitting the FROM clause is a frequent source of errors in exam environments.
Precision in Naming: Database engines are often strict regarding the spelling of table and field names. Always double-check that the names used in the query exactly match the schema provided in the problem description.
Wildcard Awareness: Use the asterisk () only when every field is required. In many practical and exam scenarios, you are asked for specific fields; listing them individually is more efficient and demonstrates a better understanding of the data requirements.
Verify Logic: When using multiple conditions in a WHERE clause, mentally test a few sample values to ensure the AND/OR logic correctly includes or excludes the intended records.
