SQL Topic: Subqueries
You might have noticed that even with a complete query, there are many questions that we can’t answer about our data without additional post, or pre, processing. In these cases, you can either make multiple queries and process the data yourself, or you can build a more complex query using SQL subqueries.
Example: General subquery
Lets say your company has a list of all Sales Associates, with data on the revenue that each Associate brings in, and their individual salary. Times are tight, and you now want to find out which of your Associates are costing the company more than the average revenue brought per Associate.
First, you would need to calculate the average revenue all the Associates are generating:
SELECT AVG(revenue_generated) FROM sales_associates;
And then using that result, we can then compare the costs of each of the Associates against that value. To use it as a subquery, we can just write it straight into the WHERE clause of the query:
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SELECT * FROM sales_associates WHERE salary > (SELECT AVG(revenue_generated) FROM sales_associates);
As the constraint is executed, each Associate’s salary will be tested against the value queried from the inner subquery.
A subquery can be referenced anywhere a normal table can be referenced. Inside a FROM clause, you can JOIN subqueries with other tables, inside a WHERE or HAVING constraint, you can test expressions against the results of the subquery, and even in expressions in the SELECT clause, which allow you to return data directly from the subquery. They are generally executed in the same logical order as the part of the query that they appear in, as described in the last lesson.
子查询可以跟一个普通的表格一样使用。FORM 从句里可以使用子查询 JOIN 其他表格，WHERE 或者 HAVING 语句里也可以对子查询的结果做判断，子查询甚至可以用在 SELECT 语句里，这时候子查询直接返回数据。（晕了晕了，没搞清楚这个 SELECT 说的哪种情况）
Because subqueries can be nested, each subquery must be fully enclosed in parentheses in order to establish proper hierarchy. Subqueries can otherwise reference any tables in the database, and make use of the constructs of a normal query (though some implementations don’t allow subqueries to use LIMIT or OFFSET).
子查询可以嵌套组合，每个子查询必须用括号包围以保证正确的层次关系。子查询可以引用数据库中的任何表，并使用普通查询的构造（尽管某些实现不允许子查询使用 LIMIT 或 OFFSET）。
A more powerful type of subquery is the correlated subquery in which the inner query references, and is dependent on, a column or alias from the outer query. Unlike the subqueries above, each of these inner queries need to be run for each of the rows in the outer query, since the inner query is dependent on the current outer query row.
Example: Correlated subquery
Instead of the list of just Sales Associates above, imagine if you have a general list of Employees, their departments (engineering, sales, etc.), revenue, and salary. This time, you are now looking across the company to find the employees who perform worse than average in their department.
For each employee, you would need to calculate their cost relative to the average revenue generated by all people in their department. To take the average for the department, the subquery will need to know what department each employee is in:
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SELECT * FROM employees WHERE salary > (SELECT AVG(revenue_generated) FROM employees AS dept_employees WHERE dept_employees.department = employees.department);
These kinds of complex queries can be powerful, but also difficult to read and understand, so you should take care using them. If possible, try and give meaningful aliases to the temporary values and tables. In addition, correlated subqueries can be difficult to optimize, so performance characteristics may vary across different databases.
When we introduced WHERE constraints in Lesson 2: Queries with constraints, the IN operator was used to test whether the column value in the current row existed in a fixed list of values. In complex queries, this can be extended using subqueries to test whether a column value exists in a dynamic list of values.
在第二节课介绍带限制条件的 WHERE 查询时，我们用 IN 来判断某一列中当前行的值是否在一个固定值列表中。在复杂的查询中，这一用法可以拓展为用子查询判断某一列的值是否存在于一个动态值列表中。语法：
When doing this, notice that the inner subquery must select for a column value or expression to produce a list that the outer column value can be tested against. This type of constraint is powerful when the constraints are based on current data.
SQL Topic: Unions, Intersections & Exceptions
When working with multiple tables, the UNION and UNION ALL operator allows you to append the results of one query to another assuming that they have the same column count, order and data type. If you use the UNION without the ALL, duplicate rows between the tables will be removed from the result.
同时操作多个表格的时候，UNION 和 UNION ALL 可以在多个查询的结果具有相同的列数、列的顺序和数据类型时把它们连接到一起。只使用 UNION 不加 ALL 的时候，结果中的重复行会被移除。
In the order of operations as defined in Lesson 12: Order of execution, the UNION happens before the ORDER BY and LIMIT. It’s not common to use UNIONs, but if you have data in different tables that can’t be joined and processed, it can be an alternative to making multiple queries on the database.
在第 12 节课提到的执行顺序里，UNION 的执行早于 ORDER BY 和 LIMIT。使用 UNION 并不是很常见，但是如果你的数据分散在不同的表格里并且不允许处理或者合并的话，这确实是一种避免多次查询的办法。
Similar to the UNION, the INTERSECT operator will ensure that only rows that are identical in both result sets are returned, and the EXCEPT operator will ensure that only rows in the first result set that aren’t in the second are returned. This means that the EXCEPT operator is query order-sensitive, like the LEFT JOIN and RIGHT JOIN.
和 UNION 类似，INTERSECT 会返回多个结果间共有的行，EXCEPT 则只会返回第一个结果中有而第二个结果中没有的行。所以 EXCEPT 是查询顺序敏感的操作，和 LEFT JOIN 和 RIGHT JOIN 一样。
Both INTERSECT and EXCEPT also discard duplicate rows after their respective operations, though some databases also support INTERSECT ALL and EXCEPT ALL to allow duplicates to be retained and returned.
INTERSECT 和 EXCEPT 也都会去掉结果中的重复行，但有的数据库支持通过 INTERSECT ALL 和 EXCEPT ALL 保留重复行。
这次真的结束了，终于。 但是这一篇翻译得很不走心，几乎是字面翻译。主要是没有例子，但是理论性的。以后使用得多了知道说的什么了有空再来来改改吧，先这样。 PEACE。
上次更新 2019-01-03 (cde1cf7)