【数据工程】8. SQL 入门教程

SQL 入门教程

基本查询：SELECT

SELECT * FROM Students;

取出所有行和列。

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条件过滤：WHERE

SELECT name, age FROM Students WHERE age > 18;

先过滤，再返回结果。

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排序：ORDER BY

SELECT name, age FROM Students ORDER BY age DESC;

按照年龄从大到小排序。

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分组：GROUP BY

SELECT class, COUNT(*) FROM Students GROUP BY class;

统计每个班级人数。

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聚合函数

SELECT MAX(age) FROM Students;   -- 最大值
SELECT MIN(age) FROM Students;   -- 最小值
SELECT AVG(age) FROM Students;   -- 平均值
SELECT SUM(age) FROM Students;   -- 总和
SELECT COUNT(*) FROM Students;   -- 行数

示例：

SELECT station, obsdate, sensor, obsvalue, site, stnownerFROM Stations NATURAL JOIN MeasurementsWHERE Stations.site = 'Murray River at Swan Hill'ORDER BY obsdate, sensor;

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HAVING 分组过滤

SELECT uosCode, COUNT(*) FROM EnrolmentsGROUP BY uosCode
HAVING COUNT(*) >= 2;

• WHERE：作用在分组之前，逐行过滤
• HAVING：作用在分组之后，整组过滤

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外连接：OUTER JOIN

三种外连接

1. LEFT OUTER JOIN：保留左表所有数据，右表不足填 NULL
2. RIGHT OUTER JOIN：保留右表所有数据，左表不足填 NULL
3. FULL OUTER JOIN：保留两边所有数据，不足的填 NULL

示例：

SELECT name AS "language"FROM language LEFT OUTER JOIN Film USING (language_id)GROUP BY name
HAVING COUNT(film_id) = 0ORDER BY name;

找到没有被任何电影使用的语言。

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总结对照

WHERE vs HAVING

• WHERE：在分组之前过滤单行数据
• HAVING：在分组之后过滤整组数据

INNER JOIN vs OUTER JOIN

• INNER JOIN：只返回两表匹配上的行
• LEFT OUTER JOIN：保留左表全部，右表不足填 NULL
• RIGHT OUTER JOIN：保留右表全部，左表不足填 NULL
• FULL OUTER JOIN：保留两表全部，不足部分填 NULL