Learning SQL statements (strong groupby and selectfrom modes)

This article describes the code for generating SQL statements using the powerful groupby and selectfrom (selectfrom) mode.

This article describes how to use powerful group by statements and use the select from (select from) mode to generate SQL statement code.

Powerful group
The Code is as follows:
Select stdname,
Isnull (sum (case stdsubject when 'chemistry 'then Result end), 0) [chemistry],
Isnull (sum (case stdsubject when 'mate' then Result end), 0) [math],
Isnull (sum (case stdsubject when 'physical 'then Result end), 0) [physical],
Isnull (sum (case stdsubject when 'China' then Result end), 0) [language]
From # student
Group by stdname

Combined with sum + case, group by can change the record (ROW) in table 1 to the field (column) in table 2 ). If there is no case in Sum, the value can only be the sum of all subjects. After case is used, the result of a certain subject is used. Then, here we use several Sum, each subject has a sum, so in Table 1, the "row" of a certain subject occupies a record is changed to a record of someone in table 2, and each subject has a field.

Use the select from (select from) mode to generate an SQL statement
The Code is as follows:
Declare @ SQL varchar (4000)
Set @ SQL = 'select stdname'
Select @ SQL = @ SQL + ', isnull (sum (case stdsubject when ''' + stdsubject + ''' then Result end), 0) [' + stdsubject + ']'
From (select distinct stdsubject from # student) as
Select @ SQL = @ SQL + 'from # student group by stdname'
Print @ SQL
Exec (@ SQL)


The Code is as follows:
Select [name] into # tmpCloumns
From tempdb. dbo. syscolumns
Where id = object_id ('tempdb. dbo. # student2 ')
And [name] <> 'stdname'
Select * from # tmpCloumns

Declare @ strSql nvarchar (800)
Select @ strSql =''
Select @ strSql = @ strSql + 'Union all' + char (10) + char (13) +
'Select [stdname], ''' + [name] + ''' as [subject], ['+ [name] +'] '+ char (10) + char (13) +
'From [# student2] '+ char (10) + char (13)
From # tmpCloumns

Select @ strSql = substring (@ strSql, 11, len (@ strSql) + 'order by stdname, [subject]'
Exec (@ strsql)

The preceding excerpted SQL code from some posts on the Internet. I saw it a long time ago. I don't know the original link.