Retrieving Data by Using Subqueries 子查詢
you should be able to:
1、Write a multiple-column subquery 多列子查詢
2、Use scalar subqueries in SQL 標量子查詢
3、Solve problems with correlated subqueries 關聯子查詢 ，子查詢子句與主查詢相互利用
4、Use the EXISTS and NOT EXISTS operators
EXISTS 子句有1條及以上的值輸出，則主查詢執行。NOT EXISTS 無值則匹配輸出。
方法：先執行主查詢的結果，再找一行值帶入子查詢來理解子查詢的意思。
5、Use the WITH clause 類似視圖
SELECT department_name, city
FROM departments
NATURAL JOIN (SELECT l.location_id, l.city, l.country_id
FROM locations l
JOIN countries c
ON(l.country_id = c.country_id)
JOIN regions
USING(region_id)
WHERE region_name = 'Europe');
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 (174, 199))
AND employee_id NOT IN (174,199);
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);
A scalar subquery expression is a subquery thatreturns exactly one column value from one row.
Scalar subqueries can be used in:
The condition and expression part of DECODE and CASE
All clauses of SELECT except GROUP BY
The SET clause and WHERE clause of an UPDATE statement
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;
select department_id, department_name,
(select count(*)
from employees e
where e.department_id = d.department_id) as emp_count
from departments d;
SELECT column1, column2, ...
FROM table1 outertable
WHERE column1 operator
(SELECT column1, column2
FROM table2
WHERE expr1 = outertable.expr2);
SELECT last_name, salary, department_id
FROM employees outer_table
WHERE salary > (SELECT AVG(salary)
FROM employees inner_table
WHERE inner_table.department_id =
outer_table.department_id);
SELECT department_id, employee_id, salary
FROM EMPLOYEES e
WHERE 1 =
(SELECT COUNT(DISTINCT salary)
FROM EMPLOYEES
WHERE e.department_id = department_id
AND e.salary <= salary)
The EXISTS operator tests for existence of rows in the results set of the subquery.
If a subquery row value is found:
The search does not continue in the inner query
The condition is flagged TRUE
If a subquery row value is not found:
The condition is flagged FALSE
The search continues in the inner query
SELECT employee_id, last_name, job_id, department_id
FROM employees outer
WHERE EXISTS ( SELECT NULL
FROM employees
WHERE manager_id =
outer.employee_id);
SELECT department_id, department_name
FROM departments d
WHERE NOT EXISTS (SELECT NULL
FROM employees
WHERE department_id = d.department_id);
Using the WITH clause, you can use the same query block in a SELECT statement when it occurs more than once within a complex query.
The WITH clause retrieves the results of a query block and stores it in the user’s temporary tablespace.
The WITH clause may improve performance.
WITH CNT_DEPT AS
(
SELECT department_id,
COUNT(1) NUM_EMP
FROM EMPLOYEES
GROUP BY department_id
)
SELECT employee_id,
SALARY/NUM_EMP
FROM EMPLOYEES E
JOIN CNT_DEPT C
ON (e.department_id = c.department_id);
WITH Reachable_From (Source, Destin, TotalFlightTime) AS
(
SELECT Source, Destin, Flight_time
FROM Flights
UNION ALL
SELECT incoming.Source, outgoing.Destin,
incoming.TotalFlightTime+outgoing.Flight_time
FROM Reachable_From incoming, Flights outgoing
WHERE incoming.Destin = outgoing.Source
)
SELECT Source, Destin, TotalFlightTime
FROM Reachable_From;