SQL学习笔记 - GROUP BY / JOIN / UNIO
最近在DataCamp上学习SQL（基于PostgreSQL）的课程，本文主要记录自己易记混的点，以便日后参考学习，不做原理讲解。
- GROUP BY（分组）一般和聚合函数一起使用，包括COUNT()，AVG()，MAX()，MIN()，SUM()；一般跟在FROM后面；SELECT语句中未出现在聚合函数里的列都要出现在GROUP BY。
2. WHERE/ GROUP BY/ HAVING/ ORDER BY 执行顺序
SELECT __ FROM __ WHERE __ GROUP BY __ (HAVING __) (ORDER BY _ ASC/DESC)
首先，WHERE将最原始记录中不满足条件的记录删除(所以应该在where语句中尽量将不符合条件的记录筛选掉，这样可以减少分组的次数)，WHERE语句不能用聚合函数；
然后，通过GROUP BY关键字对数据进行分组 ；
接着，根据HAVING关键字后面指定的筛选条件，将分组后不满足条件的记录筛选掉，（HAVING可以用聚合函数，如 HAVING AVG(col) > 10；
最后，按照ORDER BY语句进行排序。
WHER子句在聚合前先筛选记录，也就是说作用在GROUP BY和 HAVING子句前；而HAVING子句在聚合后对组记录进行筛选。
3. JOIN
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INNER JOIN / JOIN : only includes records in which the key is is both tables.
LEFT JOIN：keeps all of the records in the left table while bringing in missing values for those key field values that don't appear in the right table.
RIGHT JOIN：keeps all of the records in the right table while bringing in missing values for those key field values that don't appear in the left table.
FULL JOIN：combines a LEFT JOIN and a RIGHT JOIN, it will bring in all records from both the left and the right table and keep all of the missing values accordingly.
当用于联结两个表的字段相同时，USING等价于JOIN操作中的ON，如以下2个实例等价：
SELECT a.name, b.age FROM test AS a
JOIN test2 AS b
ON a.id = b.id;
等价于
SELECT a.name, b.age
FROM test AS a
JOIN test2 AS b
USING(id);
注：细微区别在与，USING(id) 在结果集中只会有一个id列。
4. UNION
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UNION：includes every record in both tables but DOES NOT double count those that are in both tables.（包含两个表中的每个记录，但重复的行，最终只会出现一次）
UNION ALL：includes every record in both tables and DOES replicate those are in bot tables.（包括两个表中的每个记录，并且保留重复行）
INTERSECT：results in only those records found in both of the tow tables.（交集，两个集中共同的部分）
EXCEPT：results in only those records in one table BUT NOT the other.（差异，两个集中不重复的部分）
本文首发于知乎「JessieY」
