基本 SQL

Oracle を使ううえで最優先なのは、正確な検索 SQL を書けることです。ここでは業務で頻出の SELECT / JOIN / 集計に絞って練習します。

SELECT の基本

SELECT employee_id, employee_name, department_id
FROM employees
WHERE status = 'ACTIVE'
ORDER BY employee_id;

JOIN の基本

SELECT e.employee_name, d.department_name
FROM employees e
JOIN departments d ON e.department_id = d.department_id
WHERE d.company_code = 'TOKYO';

集計と GROUP BY

SELECT department_id,
       COUNT(*) AS member_count,
       AVG(salary) AS avg_salary
FROM employees
GROUP BY department_id
HAVING COUNT(*) >= 5;

WHERE 条件

曖昧検索、期間検索、NULL 判定はバグを生みやすい箇所です。必ず期待行数を確認します。

SELECT *
FROM orders
WHERE order_date >= DATE '2026-01-01'
  AND order_date <  DATE '2026-02-01'
  AND cancelled_at IS NULL;