Introducing
Today everyone is talking about NoSQ, but SQL is still an essential
language that every programers should know enough about it as same as
JavaScript and XML. MS SQL Server is a SQL engine that enables
programmer to forward all data-related processes and worries to it, and
just interact with it by SQL. now let's have a closer look at MS SQL
Server.
Background
Because we are talking about tips here, so you need a good background of
SQL ISO and MS SQL Server, not as a DBA, but have to know enough about
DML, and DDL scripts.
Getting Started
We (Programmers) are usually have tight
contact by their IDE (Eclipse, KDEV, NetBeans,...), in fact IDE is our
best friend, but most of the times we need to persist our data and here
the first guy we can think about it is SQL.
Each developer has a favorite (best-fit) database for saving the data
(Derby, MySQL, PostgreSQL, Oracle, ... ), unfortunately most of the
times we just run some commands to the server. We imagine a database is
exact as same as ANSI SQL ISO definition (that should be, but who does
follow the rules?).
Many of us just install and configure a login account in the DB and
don't care about anything else. and what IF that database is SQL Server?
so let's get started.
Table of Contents
xp_cmdshell
How to run CMD commands in SQL Server?
easy by using xp_cmdshell.
there is a good?! guy in MS SQL called xp_cmdshell, this procedure is
used for running CMD commands, I think this is not really required while
SQL server supports Managed Beans too, BUT I have to mention this
procedure has defined before managed code introduced.
fortunately this procedure is disabled by default because of security concerns.
How to Enable xp_cmdshell in SQL Server?
this is going to be easy, at the first we need to enable advanced option with sp_configure procedure like this
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sp_configure 'show advanced options','1';
reconfigure;
then after enabling advanced options and reconfigure the system, now it's time for enabling xp_cmdshell procedure
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sp_configure 'xp_cmdshell','1';
reconfigure;
And now we have xp_cmdshell and he is ready for action, this procedure
accepts two arguments, the first one is the process name with the input
argument, and the second one is 'no_output'. 'no_output' means don't
show the response of executed application.
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exec xp_cmdshell 'echo %JAVA_HOME% , %INTELOCLSDKROOT%;'
exec xp_cmdshell 'echo %WinDir%;'
exec xp_cmdshell 'echo ''Hi Buddy'';','no_output'
The results of called commands are shown as result set in SQL (no messages).
the xp_cmdshell returns an integer number that it's result of run command
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declare @res int;
exec @res=xp_cmdshell 'C:;','no_output'
if(@res=0) begin
print ('Drive C is available :D');
end
else begin
print ('Drive C is not available !!!');
end
exec @res=xp_cmdshell 'F:;','no_output'
if(@res=0) begin
print ('Drive F is available :D');
end
else begin
print ('Drive F is not available');
end
Limitations
1: you cannot run every all applications, generally terminal
applications work without any problem but you cannot run a GUI
application like notepad.exe
2: it waits(block) until the command you run response.
3: you cannot set or get application's stream directives.
Example
now let have a good example.
How to call BCP tool is SQL Server?
using xp_cmdshell.
The BCP, its a tool installed by SQL Server, it's used for copy data
from SQL structure into user files. so because it's a executable
application and it's not a SQL command we need to call it by xp_cmdshell
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use master;
go;
create table v(id int identity(0,1),"name" varchar(64));
go
insert v values ('Arash'),('Armin'),('Aria'),('Manochehr'),('Anita'),('You');
declare @bcpComm varchar(512),@r int;
select @r=round(rand()*count(*),0) from v
set @bcpComm='BCP "select ''The lucky person is :....''+[name] from master.dbo.v where id=''"'+
convert(varchar(10),@r)+''' queryout C:\users\lucky.txt -T -c -S'+@@servername
exec @r=xp_cmdshell @bcpComm,'no_output';
if(@r=0)print 'Success, please check the C:\users\lucky.txt file';
else print('Error!');
drop table v;
Exec() and sp_executesql
How to run dynamic query in SQL Server?
easy using sp_executesql procedure
Both exec() and sp_executesql are used for running dynamic SQL commands,
just need to prepare the commands as a String object and call the
sp_executesql.
Both of exec() and sp_executesql are same, exec() is the new face of
sp_executesql has added in SQL Server V9. This procedures are enable by
default.
How to dynamically create to many tables sequentially in SQL Server?
easy, using a while loop, prepare each DDL statement and run the command string by sp_executesql procedure.
The following example creates 100 tables dynamically.
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declare @com nvarchar(1024);
declare @i int=0;
while(@i<100)begin
set @com='create table MyTable'+convert(varchar(3),@i)+'(id int,[name] varchar(64));';
exec sp_executesql @com;
set @i=@i+1;
end
select * from sys.tables where name like 'MyTable%'
xp_regwrite and xp_regread
Is it possible to access registry in SQL Server?
Yes
While it doesn't make sense, but this is possible to read an write registry with SQL Server.
I'm wondering! SQL doesn't support arrays or partial foreign keys or
hash index, or many things a good database needs, but it support access
to registry, and I don't know when does a database programmer really
need to access registry?! But HONESTLY I used this feature to much,
because of many time my application couldn't access registry, SQL helped
me out to much with this case.
Is it possible to SELECT the installation path in SQL Server?
yes, the installation path is located in registry.
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declare @ipath nvarchar(256);
exec master..xp_regread N'HKEY_LOCAL_MACHINE',N'Software\Microsoft\Microsoft SQL Server\SQL10\Setup' ,N'SQLPath',@ipath output,no_output
print @ipath
Is it possible to start SQL Agent in SQL?
yes, find the installation path, and run the SQL agent process by command.
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declare @ipath nvarchar(256);
exec master..xp_regread N'HKEY_LOCAL_MACHINE',N'Software\Microsoft\Microsoft SQL Server\SQL10\Setup' ,N'SQLPath',@ipath output,no_output
declare @sc nvarchar(1024);
set @cmdComm=N'start '+@ipath+'\binn\SQLAGENT.exe -i '+cast(@@servername as nvarchar(2048));
declare @rv int;
exec @rv=xp_cmdshell @cmdComm;
if (@rv=0)begin print 'Agent started successfully'; end
else begin print 'Agent initiating failed!'; end
Is it possible to enable TCP protocol in SQL Server by T-SQL?
Yes
how to enable SQL Authentication mode by T-SQL?
Set the (HKEY_LOCAL_MACHINE\Software\Microsoft\MSSQLServer\MSSQLServer)loginmode key to 2
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create login login_to_system_after_injection with password='Thank$SQL4Registry@ccess';
GO
exec xp_regwrite N'HKEY_LOCAL_MACHINE', N'Software\Microsoft\MSSQLServer\MSSQLServer', N'LoginMode', REG_DWORD, 2;
declare @spath nvarchar(256);
exec master..xp_regread N'HKEY_LOCAL_MACHINE',
N'Software\Microsoft\Microsoft SQL Server\Instance Names\SQL' ,N'SQL10',@spath output,no_output
declare @insRegPath nvarchar(1024)=N'Software\Microsoft\Microsoft SQL Server\' +
@spath + '\MSSQLServer\SuperSocketNetLib\Tcp';
exec xp_regwrite N'HKEY_LOCAL_MACHINE', @insRegPath, N'Enabled', REG_DWORD, 1 EXEC sys.sp_configure N'remote access', 1
GO
RECONFIGURE WITH OVERRIDE GO
shutdown
Cursor
What is cursor in SQL Server?
Cursor is SQL data-type, this is used for saving a select result in order to seek through rows.
Cursor is a really useful guy, specially when you want to fetch records
one-by-one, generally most of the database systems support cursor
because it has defined in SQL ISO.
How to declare a cursor in SQL Server?
cursors are declared as
same as variable declarations with a small difference, cursors variable
doesn't follow variable name in SQL, there is no @ symbol at first of
cursor variables.
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declare cur cursor for select * from dbo.a
declare cur scroll cursor for select * from dbo.a
Unlike explicit transactions, by default cursors are alive until you deallocate(remove) them by deallocate command
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deallocate c;
for using a cursor, you need to open the cursor, fetch information and
close it. A cursor has a pointer that points to the current row in the
result, just after you open a cursor, pointer points to nothing (before
first), so for fetching the first row, we need to fetch the next
result(move the pointer). the global @@fetch_status variable is used for
determining the current status of the cursor
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+------------------------------------------+
|--> Before the result @@fetch_status = -1 |
|-> D A T A - record @@fetch_status = 0 |
|-> D A T A - record @@fetch_status = 0 |---\
|-> D A T A - record @@fetch_status = 0 | \ A Cursor
|-> D A T A - record @@fetch_status = 0 | / Context
|-> D A T A - record @@fetch_status = 0 |---/
|-> D A T A - record @@fetch_status = 0 |
|--> After the result @@fetch_status = -1 |
+------------------------------------------+
Example
The following example tries to show folks table records in a specific
format, as above diagram is showing, when a cursor get opened, the
pointer is just before the first result, so we need to fetch next from
the cursor, then iterate the cursor by a loop.
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create table folks(id int identity(1,1),name varchar(32));
go
insert folks values('Arash'),('Pedram'),('Aidin'),('Farzad');
declare cur scroll cursor for select * from folks;
declare @id int,@name varchar(32);
open cur;
fetch next from cur into @id,@name
print('--------------------------')
while( @@FETCH_STATUS=0 )
begin
print(convert(varchar(10),@id)+space(10)+@name);
fetch next from cur into @id,@name;
end
print('--------------------------');
close cur;
deallocate cur;
drop table folks;
Always use cursor in triggers
What is a trigger?
Trigger is a functional component which is
fired implicitly by a DML or DDL event, it's used for doing alternatives
with an specific event.
When you have a trigger on a DML operation, this is possible that user
inserts or deletes more than 1 record by one insert call, so in the
trigger you should have a pattern to check all of the records affected,
that this could be done very easy by trigger.
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create table folks(id int identity(1,1),name varchar(32));
go
insert folks values('Apache'),('CodeProject'),('Google'),('sourceforge'),
('Eclipse'),('IBM'),('Oracle'),('Valve');
go
create trigger no_close_src on folks for insert as begin
declare insData cursor local for select name from inserted;
declare @name varchar(32);
open insData;
fetch next from insData into @name;
while(@@fetch_status=0) begin
set @name=lower(@name);
if ( @name='microsoft' or @name='adobe' or @name='autodesk' or @name='ea') begin
print('Error, there is no room for close source guys('+@name+') here!');
rollback transaction;
end
fetch next from insData into @name;
end
end
go
insert folks values('github'),('ea'),('RIP Sun');
select * from folks;
drop table folks;
instead of triggers
is it possible to override a DML/DDL operation in SQL Server
yes, using
instead of triggers.
Just create an
instead of trigger on target object for desired event(s), but SQL doesn't (
instead of) support it for every DDL operations of course.
How to avoiding insertion in a table without rollbacking the transaction?
Using
instead of.
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create table empty_4_ever (id int identity(0,1),_name varchar(128));
go
create trigger overrideInsert4Empty4Ever on empty_4_ever instead of trigger as begin
print('are you really sure you have inserted data?');
end
go
insert empty_4_ever values('we are wasting our time'),('have a try if you cannot believe it');
select * from empty_4_ever
As mentioned instead triggers in SQL are like method overriding in Java,
there is actual insert operation behind, but when you call the method,
you are calling the overridden method, there is just one chance to
invoke the original method/command, and this is overridden method calls
the base method itself.
this story is same for
instead of triggers in SQL server too. the following list is showing the key notes about the
instead of triggers.
- Because there is no any actual command performed by db, so there is no any transaction to commit or rollback.
- If an
instead of trigger calls its event again, it doesn't call itself again, the actual event will be invoked, for example an instead of trigger on table guys on insert, if inserts a value in table guys, it doesn't call itself again, it calles the actual insert.
- There is only ONE
instead of trigger is allowed for each event on a member
- if there are 3 trigger(for, after,
instead of ) on a member on same event(s), then the first firing trigger will be instead of trigger, other triggers will no fired UNLESS instead of trigger repeats the event.
- Use
instead of if the whole of event process should overridden, or the rate of rollbacking data is to much high.
Multi-Threading in SQL Server
Is is possible to run threads in SQL Server?
yes, but not as same as you do in Java or C, using
Service broker
What does service broker do in SQL Server?
service broker is a semi-implementation of Message Oriented Architecture(MOA)
What is MOA?
it's a pattern(architecture) designed for heterogeneous to communicate with each other. The main MOA features are listed below:
- Sending sync or async messages to the target server
- Holding events/messages in safe to prevent removing by system crashes
- Scheduling to invoking a function in future
- Sending messages across heterogeneous systems
And there is just one big disadvantage, and this is [it's unreliable]
because there is no any certain method to monitor current state of a
request in server from the client. SMS is an example of MOA.
What are a MOA's components?
Generally a MOA system contains
component listed below, but it's not a rule, MOA is architecture, is a
definition. so there is no any rule defined to force every MOA
implementations to have these components. for instance SQL Server
doesn't have Topic and agreement.
- Server: the main application, MOA implementation
- Topic & Queue: are used for holding messages
- End Point(service): the target function needs to be invoked
- Agreement: is the protocol used for communication
- Message type: the data-type for storing data, usually is typed XML
- Contract: method for marshaling and unmarshaling message in order to transfer in network
in SQL server we just need to create message type, queue, service,
contract,and. as I said create, no implementation, it's really simple.
How to execute a procedure parallel in SQL server?
Simple, as following order.
- Activate the service brokers in both client and server points(in this example both client and server are same)
- Create a procedure for handling request in server
- Create Message type and contract in both client and server points
- Create the server queue and bind the procedure to
- create the server service and bind the service to it
- create the queue in client
- create the service in client for sending messages
- create a dialog conversation(session) and send message through session
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create database _parallel
go
use _parallel
go
create proc threadStart as begin
declare @comm nvarchar(2048);
receive top(1) @comm=convert(nvarchar(2048),message_body) from messageCommPool;
exec(@comm);
end
go
alter database _parallel set enable_broker
go
create message TYPE messageComm
validation = none
go
create queue [messageCommPool] with status=on, retention=on ,activation (
procedure_name = _parallel.dbo.threadStart,
max_queue_readers = 32 , execute as owner)
go
create contract theContract
(messageComm SENT BY any)
go
create service hostService
on queue messageCommPool (theContract)
go
create queue sendQueue with status=on, retention=off;
go
create service sendService
on queue sendQueue (theContract)
go
create proc runThread (@comm nvarchar(2048)) as begin
declare @session uniqueidentifier
begin dialog conversation @session
from service sendService
to service 'hostService' on contract theContract
with encryption = off;
send on conversation @session
message type messageComm (@comm);
end
go
begin
declare @i int=0,@comm nvarchar(2048);
while(@i < 10)begin
set @comm=N'create table tableNumber'+(convert(nvarchar(2),@i))+'(id int identity(0,1),[name] char(16));'
exec runThread @comm;
set @i=@i+1;
end
end
select * from sys.tables
Service broker is a really nice feature in SQL Server, very useful, but
SQL Agent would does the same as scheduling jobs, and you have better
control on functions.
Managed Code
Is it possible to run a non-sql command in SQL?
Yes, using managed codes.
SQL Servers supports utilizing .Net platform in order to SQL-Unsupported
and complex codes, for example compressing a file, or connecting to a
URL, or killing a process.
We just need to implement the target with a CLR language(C#, VC++, VB)
and just create a procedure in SQL Server that points to the actual
code, so the .Net modules will be invoked by SQL procedure call.
This future is disabled by default, so first .Net communication should activated in the server.
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exec sp_configure 'clr enabled', 1
go
reconfigure;
go
Then code the business in .Net
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using System;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;
using System.IO;
namespace arash.blogger.example.sql{
public class StoredProcedures{
[SqlProcedure()]
public static void writeToFile(SqlString filePath, SqlString data){
BinaryWriter bw=new BinaryWriter(new FileStream(filePath.Value,FileMode.Create));
bw.Write(data.Value);
bw.Flush();
bw.Close();
SqlContext.Pipe.Send("Success! hello from PRAHS-C");
}
}
}
Then compile the class, and get the .dll or .exe file.
So there is a problem, by default, SQL Server and .Net doesn't trust the
code, because it may harm the system, so by default a managed code
cannot access external resources, and would just get some value and
return some value, but in the above code, we tried to save the content
in a FILE, so it means the code is trying to access a external resource
here. this kind of codes are called unsafe while their unsafe too.
How to run unsafe managed codes in SQL Server?
Easy, just tell the SQL be relax and trust every one, no enemies here.
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alter database mydb set trustworthy on
Okay, time to register your assembly (.dll) file in SQl server with unsafe permission.
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create assembly myAssembly authorization dbo
from 'C:\managed\managed.dll' with permission_set = unsafe
Okay and now, it's time to create a procedure and point to the actual method in the assembly.
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create procedure writeToFileProc (@name nvarchar(1024),@data nvarchar(4000)) as external name
myAssembly.[arash.blogger.example.sql.StoredProcedures].writeToFile;
and finally have a try, let's have all together
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alter database mydb set trustworthy on
go
reconfigure
go
create assembly myAssembly
from 'C:\managed\managed.dll' with permission_set = unsafe
go
create procedure writeToFile (@filePath nvarchar(1024),@data nvarchar(4000)) as external name
myAssembly.[arash.blogger.example.sql.StoredProcedures].writeToFile;
go
exec writeToFile N'C:/data.txt',N'Hello from managed codez, :D';
Except procedure, trigger, function, and data-type would be signed as managed code too, but
don't use it if you DON'T need it, SQL is heavy enough, don't make it heavier with .NET.
Tips
Use one insert command to insert multiple records
Never use 10 insert command for 10 record insert, fortunately after
years and years and years Microsoft finally provided multiple row insert
with one insert command implicitly, while just before 2008 select-union
all approach had used.
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insert guys values( 1994,'Arash M. Dehghani')
insert guys values( 1996,'Farzad B. Dehghan')
insert guys values( 1987,'Manochehr A. Dehghani')
insert guys values( 1995,'Pedram Farsi')
insert guys values( 1997,'Aidin Mirzazade')
insert guys values( 1993,'Danial Salimi')
insert guys values( 1997,'Sahand Davari')
insert guys values( 1994,'Arash M. Dehghani'),( 1996,'Farzad B. Dehghan'),( 1987,'Manochehr A. Dehghani'),
( 1995,'Pedram Farsi'),( 1997,'Aidin Mirzazade'), ( 1993,'Danial Salimi'),( 1997,'Sahand Davari')
insert guys select 1994,'Arash M. Dehghani' union all
select 1996,'Farzad B. Dehghan' union all
select 1987,'Manochehr A. Dehghani' union all
select 1995,'Pedram Farsi' union all
select 1996,'Aidin Mirzazade' union all
select 1993,'Danial Salimi' union all
select 1997,'Sahand Davari' union all
10 inserts requires 10 transaction, while 1 insert needs one, in other word, one big load is better than to many small ones.
How to prevent inserting empty string in SQL Server?
Setting a field not null doesn't mean this field doesn't accept blank
strings too! so for preventing empty strings, beside not null definition
just add a check constrain to check the empty ones.
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create table t(n varchar(64) not null ,constraint chkn check (ltrim(n)!=''));
insert t values('Danial (this is okay!)'),
(' Pedram (this is okay too!)'),
(' Code Project folks(this is still okay!)! '),
(' '),(null );
simple remove the spaces with ltrim() function, and check the value should not be as a empty string ''.
How to insert single-quote character in SQL Server?
Simple, two single-quote together means one single-quote in SQL server.
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insert messages values('Hi I''m a big fan of ''Java!'' Yeah''');
Encrypt critical data
SQL Server supports encryption/decryption algorithm in order to secure
important data, this is very useful when a database should get shared
among developers, so this ensures you no one will able access the
encrypt data UNLESS he got the password!
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create database secureGuys;
go
use secureGuys;
go;
create table DavidsFans(id bigint identity(-1,1),data varbinary(2048) not null);
go
create symmetric key theKey with algorithm=AES_256 encryption by password='P!nk-FloyD-PULSE-1994';
go
create trigger new on dbo.DavidsFans instead of insert as begin
declare C cursor local for select data from inserted;
open C;
declare @data varbinary(2048);
fetch next from C into @data;
open symmetric key theKey decryption by password='P!nk-FloyD-PULSE-1994';
while(@@fetch_status=0)begin
set @data=encryptbykey(key_guid('theKey'),@data);
insert DavidsFans values(@data);
fetch next from C into @data;
end
close symmetric key theKey;
close C; deallocate C;
end
insert DavidsFans values(cast('Me, Arash' as varbinary(2048))),
(cast('Me Again' as varbinary(2048))),
(cast('you' as varbinary(2048)));
select * from DavidsFans
go
create proc _fetch as begin
open symmetric key theKey decryption by password='P!nk-FloyD-PULSE-1994';
select *,convert(varchar(2048),DecryptBykey(data)) as 'Real Data' from DavidsFans
close symmetric key theKey;
end
go
exec _fetch
go
Indexes, friend or foe?
We all know index improves select fetching performance, but beside it's really helpful for DQL(
select)
but it's just like a daemon(foe) for DMLs, because when you have
insert, you have one insert in physical table, and one insert into each
index. and the worst part is, inserting in indexes needs to find the
exact value then insert, so this is going to be very huge process.
I AM NOT saying don't use indexes, I want to say remove or disable
indexes when you have a bulk update or bulk copy, just check the
following example and see yourself.
the following code is about a simpel table that doesn't have any
non-clustered index, we are trying to insert 10K records with randomized
value into it, how does it may take?
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create table folks(id bigint identity(0,1) primary key,
"name" char(8), age smallint,rate float);
go
declare @stratTime datetime=getdate();
declare @i int=0;
declare @rand float;
while(@i<10000) begin
set @rand=rand();
insert folks values(convert(char(8),round((@rand*9998),0)),
convert(tinyint,@rand*255),
@rand);
set @i=@i+1;
end
print('without indexes, 10K records got '+convert(varchar(64), datediff(ms,@stratTime,getDate()))+' ms :D, good enough');
as you see, without indexes , DMLs are fast as Rocket, but what if we have indexes here?
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truncate table folks
create nonclustered index nameIdx on folks("name" desc);
create nonclustered index ageIdx on folks(age asc);
create nonclustered index rateIdx on folks(rate);
go
declare @stratTime datetime=getdate();
declare @i int=0;
declare @rand float;
while(@i<10000) begin
set @rand=rand();
insert folks values(convert(char(8),round((@rand*9998),0)),
convert(tinyint,@rand*255),
@rand);
set @i=@i+1;
end
print('with indexes, 10K records got '+convert(varchar(64), datediff(ms,@stratTime,getDate()))+' ms :(');
This is not belong to SQL Server individually, this is belong to DBMSs,
so just beware when you have some big DML task, it's better to either
disable or drop indexes, then recreate/enable them after the bulk task.
SQL Injection
SQL Injection, try to run a command by values, in injection, you don't
have any access to the target database, or you don't know even what is
going on there!? what is database vendor? how many tables? how many
objects? nothing, for example, injection would be appeared in a simple
login form, for example, check this simple login form below.
Okay, now we should looking for possibilities, for example we have to
think about the possible command that checks entered username and
password, for example it could be like this.
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select userId from users where username= '{Username Value}' and password= '{password Value}';
While it never going be like above, but any way, now you have a chance
to inject your code with very last value, as above code, the password
value is last thing in the query, so what if we enter " anything' or 1=1
;-- " ? so the target generated command will be like this
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select userId from users where username= 'Fender' and password= 'anything' or 1=1 ;
So we could inject our code easily, because while password is 99.9% in
false, but 1 is equal to 1 every time, so you logged in to the system
without any problem,
but don't be surprised,
this issue has been solved in many applications today, BUT you still
have chance to search through beginners have just established their
website, and have some fun, but try to learn, instead of destroy.
Nothing more
well SQL Server is a good database, but if you ask me, I prefer
PostgreSQL, this is true, SQL Server is enterprise and has a big support
and development team, You and I as a developer except the features
related to the database from a database, while SQL supports web services
but who really is going to implement its web service with SQL Server?
it's really bad while SQL Server doesn't support arrays, modern join
commands and algebra, maps, hash indexes, inheritance, and, and and ...,
BUT it supports registry access and web services.
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