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T-SQL : Query under Conditions

Main Frame

The basic searching code is under such format, A select --from --where is the most basic structure in a query.

select A,B,C,,Z
from R1,R2,...,Rn
where F

which can be transformed into :

Thus , select means $\pi$ , and where means $\sigma​$.

If there's no limitation , means you want to get all the information from single table :

select * from TABLENAME

here , * means no limitation. or you can replace from distinct , which means canceling same tuples.

behind where you can put bool calculation , and it will select with the answer is true.

All in all , the main structure of SELECT Query is :

SELECT select_list [INTO new_table]
[FROM table_source] [WHERE search_condition]
[GROUP BY group_by_expression]--Regroup the result by condition, each group generate a single tuple in result table.
[HAVING search_condition]
[ORDER BY order_expression[ASC | DESC]]

[Notice] A single select block can be nested for another select block.

Inner function

• count([distinct|all] expression)

  Return the number of elements which is not null.

• sum(expression)

Plus every elements together and return the result.

• avg(expression)

  Return the average

• max() and min()

Additional commend

//TODO

order by :