- A query is a way to retrieve and manipulate data from a database.
- Queries can be simple or complex , depending on the criteria and operations involved.
Definition
Simple Query
A query that retrieves data based on a single condition or criterion.
Definition
Complex Query
A query that retrieves data based on multiple conditions, often using logical operators like AND, OR, and NOT.
Boolean Operators in Queries
- Boolean operators are used to combine multiple conditions in a query.
- The most common operators are:
- AND : Retrieves records that meet all specified conditions.
- OR : Retrieves records that meet at least one of the specified conditions.
- NOT : Retrieves records that do not meet a specified condition.
Example
- AND: Find all students who scored above 80 in Math and Science.
- OR : Find all students who scored above 80 in Math or Science.
- NOT : Find all students who did not score above 80 in Math.
Parameter Queries
- A parameter query allows users to input a value when the query is run.
- This makes the query dynamic and flexible.
Example
- For the query: Find all students who scored above a certain mark in Math.
- The user is prompted to enter the mark when the query is run.
Derived Fields
- A derived field is a field that is calculated from existing data in the database.
- Derived fields are often used in queries to provide additional information.
Example
- Calculate the average score of students in Math and Science.
- The derived field could be: $\text{AverageScore} = \frac{\text{(MathScore + ScienceScore)}}{2}$
Patterns and Sequences in Queries
- Queries can also be used to identify patterns and sequences in data.
- This is particularly useful in mathematical contexts.
Example
- Find all students whose scores form an arithmetic sequence (e.g., 70, 75, 80).
- The query could check if the difference between consecutive scores is constant.
