Author: admin
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Grouping Results
GROUP BY Clause: This clause groups rows that have the same values in specified columns into summary rows.Example: Explanation: This query counts the number of employees in each department. The COUNT(*) function counts all rows for each department_id.
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Aggregate Functions
Purpose: Aggregate functions allow you to perform calculations on a set of values, returning a single summary value.Common Functions: Example: Explanation: This calculates the average salary for each department, grouping results by department_id. The AVG() function computes the average, and the results will have one row for each department.
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Limiting Results
Limiting Rows: Example: Explanation: This query retrieves the first five records from the employees table. ROWNUM is a pseudo-column that indicates the order of rows returned by a query.
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Ordering Results
Example: Explanation: This command retrieves the first_name, last_name, and salary of employees, ordering the results by salary in descending order. If you wanted ascending order, you could use ASC or simply omit it, as it’s the default.
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Filtering Data with WHERE
Example Explanation: This query selects only the first_name and last_name of employees whose salary is greater than 50,000. The WHERE clause restricts the results to those meeting the condition.
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Selecting Data
Example: Explanation: This command retrieves all columns and rows from the employees table. The asterisk (*) is a wildcard that represents all columns.
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Definition of an SQL Query
SQL Query: An SQL query is a request to perform a specific operation on the database, such as retrieving data, inserting new data, updating existing records, or deleting records. In Oracle, SQL (Structured Query Language) is the standard language used to interact with the database.
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Using WITH READ ONLY
The WITH READ ONLY clause prevents any DML operations (INSERT, UPDATE, DELETE) on the view. This is useful for views intended solely for data retrieval. Example Explanation: This view can be queried but cannot be modified. It provides a safeguard against accidental data changes, ensuring data integrity.