What does SQL clause “GROUP BY 1” mean?

问题

Someone sent me a SQL query where the GROUP BY clause consisted of the statement: GROUP BY 1.

This must be a typo right? No column is given the alias 1. What could this mean? Am I right to assume that this must be a typo?

回答1:

It means to group by the first column regardless of what it's called. You can do the same with ORDER BY.

回答2:

SELECT account_id, open_emp_id
         ^^^^        ^^^^
          1           2

FROM account
GROUP BY 1;

In above query GROUP BY 1 refers to the first column in select statement which is account_id.

You also can specify in ORDER BY.

Note : The number in ORDER BY and GROUP BY always start with 1 not with 0.

回答3:

In addition to grouping by the field name, you may also group by ordinal, or position of the field within the table. 1 corresponds to the first field (regardless of name), 2 is the second, and so on.

This is generally ill-advised if you're grouping on something specific, since the table/view structure may change. Additionally, it may be difficult to quickly comprehend what your SQL query is doing if you haven’t memorized the table fields.

If you are returning a unique set, or quickly performing a temporary lookup, this is nice shorthand syntax to reduce typing. If you plan to run the query again at some point, I’d recommend replacing those to avoid future confusion and unexpected complications (due to scheme changes).

回答4:

It will group by first field in the select clause

回答5:

It will group by the column position you put after the group by clause.

for example if you run 'SELECT SALESMAN_NAME, SUM(SALES) FROM SALES GROUP BY 1' it will group by SALESMAN_NAME.

One risk on doing that is if you run 'Select *' and for some reason you recreate the table with columns on a different order, it will give you a different result than you would expect.

回答6:

That means sql group by 1st column in your select clause, we always use this GROUP BY 1 together with ORDER BY 1, besides you can also use like this GROUP BY 1,2,3.., of course it is convenient for us but you need to pay attention to that condition the result may be not what you want if some one has modified your select columns, and it's not visualized

来源：https://stackoverflow.com/questions/7392730/what-does-sql-clause-group-by-1-mean

标签

mysql

sql

group-by