SQLite“COUNT（*）”被认为是有害的？

Question

考虑下面简单的例子：

prompt% sqlite3 test.db 
sqlite> create table employee (
      employee_id integer  primary key, 
      first_name varchar2(32) not null, 
      last_name  varchar2(32) not null 
     ); 

sqlite> insert into employee (first_name, last_name) values ('Bill', 'Smith'); 
sqlite> insert into employee (first_name, last_name) values ('Sally', 'Jones'); 
sqlite> insert into employee (first_name, last_name) values ('Bill', 'Jones'); 

sqlite> select first_name, count(*) from employee; 

会有什么结果呢？

天真的可能会认为这将是：

Bill|2 
Sally|1 

但经历会发现，SELECT查询缺少“GROUP BY子句。甲骨文，当此查询提出将，其实，扔了一个错误：

SQL ERROR: ORA-00937: not a single-group group function 

SQLite的，但是，不抱怨，而是产生：

Bill|3 

这似乎假我。 ..我想，显示行的总数可能是有意义的，但只要选择最后一个'first_name'似乎是任意的和潜在的危险。

这是一个错误或功能，我只是不明白？ SQLite不提供类似的安全网有什么原因吗？

Answer 1

这是所有documented：

If the SELECT statement is an aggregate query without a GROUP BY clause, then each aggregate expression in the result-set is evaluated once across the entire dataset. Each non-aggregate expression in the result-set is evaluated once for an arbitrarily selected row of the dataset. The same arbitrarily selected row is used for each non-aggregate expression...

The single row of result-set data created by evaluating the aggregate and non-aggregate expressions in the result-set forms the result of an aggregate query without a GROUP BY clause. An aggregate query without a GROUP BY clause always returns exactly one row of data, even if there are zero rows of input data.

简单地说，有没有安全网。如果您需要，请不要忘记GROUP BY。