如何高效地查詢兩列上的多對多關係

Question

我已經創建了這三個表，表示醫生和醫生的專業之間的多對多關係。

醫生可以有許多專業，許多醫生可以有一定的專業。

CREATE TABLE doctors (
    doctor_id int(11) NOT NULL AUTO_INCREMENT, 
    firstname varchar(30) NOT NULL, 
    lastname varchar(30) NOT NULL, 
    area varchar(50) NOT NULL, 
    PRIMARY KEY (doctor_id) 
); 

CREATE TABLE has_specialty (
    doctor_id int(11) NOT NULL, 
    specialty_id int(11) NOT NULL, 
    PRIMARY KEY (doctor_id,specialty_id), 
    FOREIGN KEY (doctor_id) REFERENCES doctors (doctor_id), 
    FOREIGN KEY (specialty_id) REFERENCES specialties (specialty_id) 
); 

CREATE TABLE IF specialties (
    specialty_id int(11) NOT NULL AUTO_INCREMENT, 
    specialty varchar(254) NOT NULL UNIQUE, 
    PRIMARY KEY (specialty_id) 
); 

• 我要的是找出所有有專業的「H」，並在區域「B」的醫生。

因此，例如，讓我們說我們有這個數據庫：

DOCTORS 
+-----------+-----------+----------+----------+ 
| doctor_id | firstname | lastname | area  | 
+-----------+-----------+----------+----------+ 
|   1 | Virginia | Clark | A  | 
|   2 | Jane  | Brown | B  | 
|   3 | Adam  | Harris | D  | 
|   4 | Anthony | Rogers | D  | 
|   5 | Paula  | Lopez | B  | 
|   6 | Jerry  | Howard | A  | 
|   7 | Willie | Nelson | C  | 
|   8 | Juan  | Perry | A  | 
|   9 | Victor | Allen | B  | 
+-----------+-----------+----------+----------+ 

SPECIALTIES 
+--------------+-----------+ 
| specialty_id | specialty | 
+--------------+-----------+ 
| 1   | A   | 
| 2   | B   | 
| 3   | C   | 
| 4   | D   | 
| 5   | E   | 
| 6   | F   | 
| 7   | G   | 
| 8   | H   | 
| 9   | I   | 
+--------------+-----------+ 

HAS_SPECIALTY 
+-----------+--------------+ 
| doctor_id | specialty_id | 
+-----------+--------------+ 
| 1   | 4   | 
| 1   | 6   | 
| 1   | 8   | 
| 2   | 3   | 
| 2   | 8   | 
| 3   | 1   | 
| 3   | 4   | 
| 3   | 5   | 
| 4   | 4   | 
| 5   | 8   | 
| 5   | 9   | 
| 6   | 2   | 
| 6   | 7   | 
| 7   | 9   | 
| 8   | 4   | 
| 9   | 2   | 
| 9   | 3   | 
+-----------+--------------+ 

結果應該是：

+-----------+-----------+----------+----------+ 
| doctor_id | firstname | lastname | area  | 
+-----------+-----------+----------+----------+ 
|   2 | Jane  | Brown | B  | 
|   5 | Paula  | Lopez | B  | 
+-----------+-----------+----------+----------+ 

Answer 1

你可以做到這一點簡單INNER JOIN的：

SELECT d.doctor_id, d.firstname,d.lastname,d.area  
FROM doctors d 
INNER JOIN has_specialty hs ON d.doctor_id = hs.doctor_id 
INNER JOIN specialties s ON hs.specialty_id = s.specialty_id 
WHERE s.specialty = 'H' AND d.area = 'B'; 

sqlfiddle demo