SQL：连接同一个表中的 3 个选择查询（内连接和左连接）

Question

我正在使用 MySQL，但遇到了问题。 我需要对同一个表上执行的 3 个查询的输出进行交叉。就是这张桌子：

CREATE TABLE posting (
    doc integer NOT NULL,
    word varchar(30) NOT NULL,
    freq integer NOT NULL,
    primary key (doc, word));

然后我插入了一些值。

SELECT * FROM posting;

+-----+----------------+------+
| doc | word           | freq |
+-----+----------------+------+
|   1 | app            |   40 |
|   1 | classification |   20 |
|   1 | context        |   30 |
|   1 | information    |   15 |
|   1 | mobile         |   20 |
|   2 | app            |   40 |
|   2 | context        |   30 |
|   2 | discovery      |   15 |
|   2 | mobile         |   20 |
|   2 | recommandation |   30 |
|   2 | wall           |   15 |
|   3 | app            |   40 |
|   3 | discovery      |   10 |
|   3 | ideal          |   10 |
|   3 | mobile         |   20 |
|   3 | search         |   30 |
|   3 | server         |   25 |
|   4 | app            |   40 |
|   4 | killer         |   25 |
|   4 | mobile         |   20 |
|   4 | recommandation |   10 |
|   4 | search         |   30 |
|   5 | app            |   40 |
|   5 | beyond         |   20 |
|   5 | mobile         |   20 |
|   5 | model          |   15 |
|   5 | service        |   20 |
|   5 | share          |   30 |
|   5 | store          |   15 |
+-----+----------------+------+

我做了一些基本的询问。

SELECT DISTINCT doc FROM posting WHERE word LIKE 'mobile' AND freq >= 20;
+-----+
| doc |
+-----+
|   1 |
|   2 |
|   3 |
|   4 |
|   5 |
+-----+

所以集合为 M = {1, 2, 3, 4, 5}

SELECT DISTINCT doc FROM posting WHERE word LIKE 'context' AND freq >= 20;
+-----+
| doc |
+-----+
|   1 |
|   2 |
+-----+

所以集合是 C = {1, 2}

SELECT DISTINCT doc FROM posting WHERE word LIKE 'search' AND freq >= 20;
+-----+
| doc |
+-----+
|   3 |
|   4 |
+-----+

所以集合是 S = {3, 4}

SELECT DISTINCT doc FROM posting WHERE word LIKE 'app' AND freq >= 20;
+-----+
| doc |
+-----+
|   1 |
|   2 |
|   3 |
|   4 |
|   5 |
+-----+

所以集合是 A = {1, 2, 3, 4, 5}

现在我需要进行 2 个查询。

将第一个、第二个和第三个相交。 --> M ∩ C ∩ S = {1,2,3,4,5} ∩ {1,2} ∩ {3,4} = {}

(SELECT DISTINCT doc FROM posting WHERE word LIKE 'mobile' AND freq >= 20
INNER JOIN
posting WHERE word LIKE 'context' AND freq >= 20)
INNER JOIN
posting WHERE word LIKE 'search' AND freq >= 20;

将第一组、第四组和第二组的互补组相交。 --> M ∩ A ∩ 互补C = {1,2,3,4,5} ∩ {1,2,3,4,5} ∩ {3,4,5} = {3,4,5}

(SELECT DISTINCT doc FROM posting WHERE word LIKE 'mobile' AND freq >= 20
INNER JOIN
posting WHERE wordLIKE 'apps' AND freq >= 20)
LEFT JOIN
posting WHERE word LIKE 'context' AND freq >= 20;

Answer 1

对于单个表中多个条件的交叉结果，

INNER JOIN

与表本身一起使用，基于匹配的文档 ID 和每个交叉集的特定条件：

SELECT DISTINCT m.doc 
FROM posting m
INNER JOIN posting c ON m.doc = c.doc AND c.word = 'context' AND c.freq >= 20
INNER JOIN posting s ON m.doc = s.doc AND s.word = 'search' AND s.freq >= 20
WHERE m.word = 'mobile' AND m.freq >= 20;

要查找给定条件的互补集，请结合使用

LEFT JOIN

和

WHERE ... IS NULL

检查来查找与条件不匹配的条目：

SELECT DISTINCT m.doc 
FROM posting m
INNER JOIN posting a ON m.doc = a.doc AND a.word = 'app' AND a.freq >= 20
LEFT JOIN posting c ON m.doc = c.doc AND c.word = 'context' AND c.freq >= 20
WHERE m.word = 'mobile' AND m.freq >= 20 AND c.doc IS NULL;