NULL. , .. - .. . SQL Null- ( -). - Null, -, . , . -. - Null . . . - , .. - -. - :
< > Is [Not] Null.
-, Is Null - , Is Not Null , .
:
1) , .
Select *
From P
Where Status Is Not Null
Where Not Status Is Null
2) ,
Select *
From P
Where Status Is Like Null
18. SQL. (+ )
SQL , , - .
- Group by, . - , - , . - - .
:
1) Count - - ,
2) Sum - .
3) Avg - . .
4) Min - . .
5) Max - . .
- Select - - .
Sum vg - , Min, Max, Count - , .
Min, Max - - .
:
Count (*) |
{Avg | Max | Min | Sum | Count}
(Distinct <>)
{Avg | Max | Min | Sum }
(Distinct <>)
([All] < >)
Count (*) - - -, - . - , - Count , Null. Count (*) , Null- - , -.
- distinct, - Null- , - - . - - .
|
|
. - Distinct ( All), - - - . Null- - . - Count.
:
1) - ., .
Select Discipl As , Count (*) As
From R1
Group by Discipl 1.
. .
2) - :
Select Count (*) As
From R1
Where Mark > 3
- - , . .
.. .
3) - .
Select Gr, Discipl, Count(*), Avg (Mark)
From R1, R2
Where R1.FIO = R2.FIO and Mark >3
Group by Gr, Discipl 2.
- Where, .. - , - . , Having.
Having , , Group by. Group by. , - , 1 . Having , Wher. . Having , Group By. - - , .
4) , 1 .
Select Gr, Discipl
From R1, R2
Where R1.FIO = R2.FIO and Mark <=3
Group By Gr, Discipl
Having Count (*)>1 : Gr Discipl
Having Avg (Mark)>6 111
5) ,
Select Num_D
From PD
Group By Num_D
Having Count (Distinct Num_P)>1
Order By -, .. .
:
Order By {< > | <c>} [Asc | Desc]
- - - .
Asc ( )
Desc
, :
1. Union Union All
2. Select ,
Order By - .
Order By - Group By , - Group By.
|
|
6) - - , , ,
Select Gr As , Discipl As , R1.FIO As , Mark As
From R1, R2
Where R1.FIO=R2.FIO
Order By Gr, Discipl, R1.FIO 3.
7) - , -:
Select Discipl As , Count(*) As
From R1
Group By Discipl
Order By 2 4.