oracle 數(shù)據(jù)庫中的 having 子句用于從分組數(shù)據(jù)中篩選組,其應(yīng)用場景包括:基于聚合值如平均值或總和過濾組;比較不同組之間的聚合值;篩選多級分組中的組;在子查詢中用于篩選外層查詢中的數(shù)據(jù)。
oracle 數(shù)據(jù)庫中 HAVING 子句的應(yīng)用場景
什么是 HAVING 子句?
HAVING 子句用于在 Oracle 數(shù)據(jù)庫中從分組數(shù)據(jù)中篩選組。它類似于 WHERE 子句,但不是根據(jù)單個(gè)行數(shù)據(jù)進(jìn)行篩選,而是根據(jù)組中數(shù)據(jù)的聚合值進(jìn)行篩選。
HAVING 子句的應(yīng)用場景:
1. 過濾聚合值
- 檢查特定組中是否滿足聚合條件(例如,平均值是否大于某個(gè)閾值)。
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示例:`select department_id, AVG(salary) AS avg_salary
FROM employees GROUP BY department_id HAVING AVG(salary) > 50000;`
2. 檢查組之間的關(guān)系
- 比較不同組的聚合值(例如,檢查哪個(gè)組的總銷售額最高)。
-
示例:`SELECT department_id, SUM(sales) AS total_sales
FROM sales GROUP BY department_id HAVING SUM(sales) > ( SELECT SUM(sales) FROM sales WHERE department_id = 'Marketing' );`
3. 過濾層疊分組
- 在多級分組中,HAVING 子句可用于篩選內(nèi)層或外層分組。
-
示例:`SELECT dept_id, job_id, SUM(salary) AS total_salary
FROM employees GROUP BY dept_id, job_id HAVING SUM(salary) > 100000;`
4. 在子查詢中使用
- HAVING 子句可在子查詢中使用,以篩選外層查詢中的數(shù)據(jù)。
-
示例:`SELECT employee_id
FROM employees WHERE department_id IN ( SELECT department_id FROM departments GROUP BY department_id HAVING AVG(salary) > 60000 );`