Common Table Expressions in Oracle Pl/Sql and Hive explained and Compared

by | Jun 6, 2024 | SQL | 0 comments

Common Table Expressions (CTEs) are a useful feature in SQL for simplifying complex queries and improving readability. Both Oracle PL/SQL and Apache Hive support CTEs, although there may be slight differences in their syntax and usage.

Common Table Expressions in Oracle PL/SQL

In Oracle, CTEs are defined using the WITH clause. They are used to create a temporary result set that can be referenced within the main query.

Syntax:

WITH cte_name AS (
SELECT query
)
SELECT columns
FROM cte_name
WHERE condition;

Example Usage:

WITH department_totals AS (
SELECT department_id, SUM(salary) AS total_salary
FROM employees
GROUP BY department_id
)
SELECT d.department_id, d.department_name, t.total_salary
FROM departments d
JOIN department_totals t ON d.department_id = t.department_id;

In this example:

  • The CTE department_totals calculates the total salary for each department.
  • The main query then joins the departments table with the department_totals CTE to display the department name and total salary.

Common Table Expressions in Apache Hive

In Hive, CTEs are also defined using the WITH clause. They function similarly to those in Oracle, allowing you to create temporary result sets for use within a larger query.

Syntax:

WITH cte_name AS (
SELECT query
)
SELECT columns
FROM cte_name
WHERE condition;

Example Usage:

WITH department_totals AS (
SELECT department_id, SUM(salary) AS total_salary
FROM employees
GROUP BY department_id
)
SELECT d.department_id, d.department_name, t.total_salary
FROM departments d
JOIN department_totals t ON d.department_id = t.department_id;

In this example:

  • The syntax and functionality are essentially the same as in Oracle PL/SQL.
  • The CTE department_totals is used to calculate the total salary for each department.
  • The main query joins the departments table with the department_totals CTE to display the department name and total salary.

Key Points

  • Scope: CTEs in both Oracle and Hive are limited to the scope of the query they are defined in. Once the query execution is complete, the CTEs are discarded.
  • Multiple CTEs: You can define multiple CTEs in a single query by separating them with commas.
  • Recursion: Oracle supports recursive CTEs, which allow a CTE to refer to itself. Hive also supports recursive CTEs starting from version 3.1.0.

Example of Multiple CTEs:

WITH
cte1 AS (
SELECT department_id, SUM(salary) AS total_salary
FROM employees
GROUP BY department_id
),
cte2 AS (
SELECT department_id, COUNT(*) AS employee_count
FROM employees
GROUP BY department_id
)
SELECT c1.department_id, c1.total_salary, c2.employee_count
FROM cte1 c1
JOIN cte2 c2 ON c1.department_id = c2.department_id;

In both Oracle and Hive, this example:

  • Defines two CTEs: cte1 and cte2.
  • The main query joins these two CTEs to display the department ID, total salary, and employee count for each department.

CTEs are a powerful tool in both Oracle PL/SQL and Hive for breaking down complex queries, making them easier to read and maintain.

Written by HintsToday Team

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