oracle_高級子查詢
①子查詢
子查詢是嵌套在 SQL 語句中的另一個SELECT 語句
子查詢 (內查詢) 在主查詢執行之前執行
主查詢(外查詢)使用子查詢的結果
②多列子查詢
主查詢與子查詢返回的多個列進行比較
where (column1,column2) in (子查詢)
例：查詢與141號或174號員工的manager_id和department_id相同的其他員工的employee_id, manager_id, department_id
多列子查詢中的比較分為兩種:
1)成對比較
SELECT employee_id, manager_id, department_id
FROM employees
WHERE (manager_id, department_id) IN
(SELECT manager_id, department_id
FROM employees
WHERE employee_id IN (141,174))
AND employee_id NOT IN (141,174);
2)不成對比較
SELECT employee_id, manager_id, department_id
FROM employees
WHERE manager_id IN (SELECT manager_id
FROM employees
WHERE employee_id IN (174,141))
AND department_id IN (SELECT department_id
FROM employees
WHERE employee_id IN (174,141))
AND employee_id NOT IN(174,141);
③在 FROM 子句中使用子查詢
例：返回比本部門平均工資高的員工的last_name, department_id, salary及平均工資
1)select last_name,department_id,salary,
(select avg(salary)from employees e3 where e1.department_id = e3.department_id group by department_id) avg_salary
from employees e1
where salary >
(select avg(salary)
from employees e2
where e1.department_id = e2.department_id
group by department_id
)
2)SELECT a.last_name, a.salary, a.department_id, b.salavg
FROM employees a, (SELECT department_id,
AVG(salary) salavg
FROM employees
GROUP BY department_id) b
WHERE a.department_id = b.department_id
AND a.salary > b.salavg;
④單列子查詢表達式
單列子查詢表達式是在一行中只返回一列的子查詢
Oracle8i 只在下列情況下可以使用, 例如:
SELECT 語句 (FROM 和 WHERE 子句)
INSERT 語句中的VALUES列表中
Oracle9i中單列子查詢表達式可在下列情況下使用:
DECODE 和 CASE
SELECT 中除 GROUP BY 子句以外的所有子句中
1)在 CASE 表達式中使用單列子查詢
例：顯式員工的employee_id,last_name和location。其中，
若員工department_id與location_id為1800的department_id相同，則location為’Canada’,其余則為’USA’。
SELECT employee_id, last_name,
(CASE WHEN department_id = (SELECT department_id FROM departments WHERE location_id = 1800) THEN 'Canada'
ELSE 'USA' END
) location
FROM employees;
2)在 ORDER BY 子句中使用單列子查詢
例：查詢員工的employee_id,last_name,要求按照員工的department_name排序
SELECT employee_id, last_name
FROM employees e
ORDER BY (SELECT department_name
FROM departments d
WHERE e.department_id = d.department_id);
⑤相關子查詢
相關子查詢按照一行接一行的順序執行，主查詢的每一行都執行一次子查詢
SELECT column1, column2, ...
FROM table1 outer
WHERE column1 operator (SELECT colum1, column2
FROM table2
WHERE expr1 = outer.expr2);
例：查詢員工中工資大於本部門平均工資的員工的last_name,salary和其department_id
SELECT last_name, salary, department_id
FROM employees outer
WHERE salary >(SELECT AVG(salary)
FROM employees
WHERE department_id = outer.department_id)
例：若employees表中employee_id與job_history表中employee_id相同的數目不小於2，輸出這些相同id的員工的employee_id,last_name和其job_id
SELECT e.employee_id, last_name,e.job_id
FROM employees e
WHERE 2 <= (SELECT COUNT(*)
FROM job_history
WHERE employee_id = e.employee_id);
⑥EXISTS 操作符
EXISTS 操作符檢查在子查詢中是否存在滿足條件的行
1.如果在子查詢中存在滿足條件的行:
不在子查詢中繼續查找
條件返回 TRUE
2.如果在子查詢中不存在滿足條件的行:
條件返回 FALSE
繼續在子查詢中查找
例：查詢公司管理者的employee_id,last_name,job_id,department_id信息
SELECT employee_id, last_name, job_id, department_id
FROM employees outer
WHERE EXISTS ( SELECT 'X'
FROM employees
WHERE manager_id =
outer.employee_id);
例：查詢departments表中，不存在於employees表中的部門的department_id和department_name
SELECT department_id, department_name
FROM departments d
WHERE NOT EXISTS (SELECT 'X'
FROM employees
WHERE department_id
= d.department_id);
⑦相關更新
UPDATE table1 alias1
SET column = (SELECT expression
FROM table2 alias2
WHERE alias1.column =
alias2.column);
使用相關子查詢依據一個表中的數據更新另一個表的數據
⑧相關刪除
DELETE FROM table1 alias1
WHERE column operator
(SELECT expression
FROM table2 alias2
WHERE alias1.column = alias2.column);
⑨WITH 子句
使用 WITH 子句, 可以避免在 SELECT 語句中重復書寫相同的語句塊
WITH 子句將該子句中的語句塊執行一次並存儲到用戶的臨時表空間中
使用 WITH 子句可以提高查詢效率
例：查詢公司中各部門的總工資大於公司中各部門的平均總工資的部門信息
WITH dept_costs AS (
SELECT d.department_name, SUM(e.salary) AS dept_total
FROM employees e, departments d
WHERE e.department_id = d.department_id
GROUP BY d.department_name),
avg_cost AS (
SELECT SUM(dept_total)/COUNT(*) AS dept_avg
FROM dept_costs)
SELECT *
FROM dept_costs
WHERE dept_total >
(SELECT dept_avg
FROM avg_cost)
ORDER BY department_name;
