SQL Server subquery Error

SQL Server in contrast to Oracle, if the subquery involved, then the corresponding table name, field must be named an alias to execute. There are two main types of situations:

1. Query SQL inside only a simple subquery does not contain grouping, sum wait Eg:elect * FROM (select com_id from company) so it will error, must add an alias, the layer of SELECT * from company here can add alias, You can also not add the correct: SELECT * FROM (select com_id to Company) a

2. Query SQL contains grouping, summation, or other functions when eg:select * FROM (select com_id, Count (com_id) from company Group by COM_ID) a here if you alias the outermost SQL , or the error will be added to company alias B select * FROM (select Com_id,count (com_id) to Company B Group by COM_ID) A that's not going to work because actually the count () function You can actually do it as a subquery, so you want to add the alias to count () correctly: SELECT * FROM (select com_id, COUNT (com_id) c from Company B Group by com_id) a