相当于关系代数中的 GroupBy 和having 子句
我正在做一项作业，必须将 SQL 查询转换为关系代数查询。我在转换 group by 子句时陷入困境。
谁能告诉我如何用关系代数编写 group by 子句？
例如：
SELECT job, sal
FROM emp
GROUP BY job
;
谢谢！
I was doing an assignment where I had to convert SQL queries into Relational Algebra queries. I got stuck in converting the group by clause.
Could anyone please tell me how the group by clause can be written in relational algebra?
e.g.:
SELECT job, sal
FROM emp
GROUP BY job
;
Thanks!
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注意您想要获得工资总和，在 教程 D 中：
注意聚合是不是关系运算符，因此不会构成关系代数的一部分。
至于
HAVING，是历史异常吗？在 SQL-92 标准之前，不可能在FROM子句（也称为派生表）中编写SELECT表达式，即您必须在一个中完成所有工作选择表达式。由于 SQL 严格的求值顺序，在对WHERE子句求值后，聚合值并不存在，即不可能应用基于聚合值的限制。HAVING的引入就是为了解决这个问题。但即使有了
HAVING，在引入派生表之前，SQL 在 Codd 方面仍然是不完整的。派生表呈现为HAVING冗余，但使用HAVING仍然很流行（如果 Stackoverflow 可以作为参考的话）：人们似乎仍然喜欢使用单个SELECTHAVING相比，如果可能，SQL 前面提到的关于计算顺序的严格性（投影在SELECT表达式中最后执行）使得派生表的使用相当冗长。Noting you want to get the sum of salary, in Tutorial D:
Note aggregation is not a relational operator, hence will not form part of a relational algebra.
As for
HAVING, is it a historical anomaly. Before the SQL-92 Standard, it was not possible to writeSELECTexpressions in theFROMclause (a.k.a derived tables) i.e. you had to do all work in oneSELECTexpression. Because of SQL's rigid evaluation order, the aggregate value doesn't come into existence after theWHEREclause has been evaluated i.e. it was impossible apply restriction based on aggregated values.HAVINGwas introduced to address this problem.But even with
HAVING, SQL remained relationally incomplete as regards Codd's until derived tables had been introduced. Derived tables renderedHAVINGredundant but usingHAVINGis still popular (if Stackoverflow is anything to go by): folk still seem to like to use a singleSELECTwhere possible and SQL's aforementioned rigidity as regards evaluations order (projection is performed last in aSELECTexpression) makes derived table usage quite verbose when compared toHAVING.首先，您的查询是错误的，除非您使用聚合，否则您无法选择未分组的内容。我假设你想得到萨尔的总和。
工作 F 总和（sal），工作（emp）。
First of all your query is wrong you cannot select something that you did not group unless you use aggregation. I assume you want to get sum of the sal.
job F sum(sal), job(emp).