Friday, September 11, 2009

FAQ: What is optimizer in SQL Server?

Unlike procedural language SQL Statements does not state the exact steps that database engine should follow to extract the data or manipulate the data. In procedural language the statements are ran as per the sequence they have written. But in SQL , database engine must analyse the statement to determine the most efficient way to extract/manipulate data. This process is called optimization and the component which does this process is called Optimizer.

The input to the optimizer contains , Query , schema (table/indexes definitions) and the statistics available.

The output from the optimizer is query execution plan.



The SQL Server query optimizer is a cost-based optimizer. Each possible execution plan has an associated cost in terms of the amount of computing resources used. The query optimizer must analyze the possible plans and choose the one with the lowest estimated cost. Some complex SELECT statements have thousands of possible execution plans. In these cases, the query optimizer does not analyze all possible combinations. Instead, it uses complex algorithms to find an execution plan that has a cost reasonably close to the minimum possible cost.

Sunday, September 6, 2009

FAQ : What is Optimizing SQL statement .

Unlike procedural language SQL Statements does not state the exact steps that database engine should follow to extract the data or manipulate the data. In procedural language the statements are ran as per the sequence they have written. But in SQL , database engine must analyse the statement to determine the most efficient way to extract/manipulate data. This process is called optimization.

Wednesday, September 2, 2009

FAQ : What is the equivalent of SELECT * INTO of SQL Server in Oracle

SQL Server
Select * into TargettableName from sourceTableName

Oracle
Create table targetTableName as Select *From sourceTableName

FAQ : What is Packages in Oracle and its equivalent in SQL Server

A package is a schema object that groups logically related PL/SQL types, variables, and subprograms. Packages usually have two parts, a specification (spec) and a body; sometimes the body is unnecessary. The specification is the interface to the package. It declares the types, variables, constants, exceptions, cursors, and subprograms that can be referenced from outside the package. The body defines the queries for the cursors and the code for the subprograms.

There is no equivalent in SQL Server yet.

FAQ : How to declare a variable in SQL Server (T-SQL) and in Oracle PL/SQL

SQL Server

Variables are declared in the body of a batch or procedure with the DECLARE Statement and are assigned to values with either a SET or SELECT Statement.After declaration all variables are initialized as NULL
Variables are often used in batch or procedure /functions as counters for WHILE, LOOP or for an IF..ELSE block. Variables can be used only in expression, not in the place of object names or key words (in that case you need to use Dynamic SQL. See Dynamic SQL for more info).

Types of Variable : Three types of variables are available
• @local variable
• @Cursor_Variable_Name
• @table_variable_name

Declare a variable :
DECLARE @VariableName datatype : Variable prefixed with @ symbol.
Eg. DECLARE @Count INT
DECLARE @Name varchar(50), @Address varchar(200)

Seting /initialising a variable :

SET @Name=’Madhu’
SET @Count=100
Initialise multiple variable using Select statement :

SET can be used for only single variable where as SELECT can be used for multiple variable in a single statement

SELECT @Name=’Madhu’ , @Count=0 , @Address=’XYZ’

WHILE(@i<@Count)
BEGIN
SET @i=@i+1
PRINT @i
END

Oracle (PL/SQL) : Declaring Variables
In oracle, Variables can have any SQL datatype, such as CHAR, DATE, or NUMBER, or a PL/SQL-only datatype, such as BOOLEAN or PLS_INTEGER. For example, assume that you want to declare variables for part data, such as part_no to hold 6-digit numbers and in_stock to hold the Boolean value TRUE or FALSE. You declare these and related part variables as shown in Example 1. Note that there is a semi-colon (;) at the end of each line in the declaration section.

Example 1–1. Declaring Variables in PL/SQL
DECLARE
part_no NUMBER(6);
part_name VARCHAR2(20);

You can also declare nested tables, variable-size arrays (varrays for short), and records using the TABLE, VARRAY, and RECORD composite datatypes.

Assigning Values to a Variable
You can assign values to a variable in three ways. The first way uses the assignment operator (:=), a colon followed by an equal sign, as shown in Example 2. You place the variable to the left of the operator and an expression, including function calls, to the right. Note that you can assign a value to a variable when it is declared.

Example 1–2 Assigning Values to Variables With the Assignment Operator
DECLARE
wages NUMBER;
hours_worked NUMBER := 40;
emp_rec1 employees%ROWTYPE;
country := UPPER('Canada');


The second way to assign values to a variable is by selecting (or fetching) database values into it. In Example 1–3, 10% of an employee's salary is selected into the bonus variable. Now you can use the bonus variable in another computation or insert its value into a database table.

Example 1–3 Assigning Values to Variables by SELECTing INTO

DECLARE
bonus NUMBER(8,2);
emp_id NUMBER(6) := 100;
BEGIN
SELECT salary * 0.10 INTO bonus FROM employees
WHERE employee_id = emp_id;
END;

The third way to assign a value to a variable is by passing it as an OUT or IN OUT parameter to a subprogram, and then assigning the value inside the subprogram.

FAQ : How to declare a variable in SQL Server (T-SQL) and in Oracle PL/SQL

SQL Server

Variables are declared in the body of a batch or procedure with the DECLARE Statement and are assigned to values with either a SET or SELECT Statement.After declaration all variables are initialized as NULL
Variables are often used in batch or procedure /functions as counters for WHILE, LOOP or for an IF..ELSE block. Variables can be used only in expression, not in the place of object names or key words (in that case you need to use Dynamic SQL. See Dynamic SQL for more info).

Types of Variable : Three types of variables are available
• @local variable
• @Cursor_Variable_Name
• @table_variable_name

Declare a variable :
DECLARE @VariableName datatype : Variable prefixed with @ symbol.
Eg. DECLARE @Count INT
DECLARE @Name varchar(50), @Address varchar(200)

Seting /initialising a variable :

SET @Name=’Madhu’
SET @Count=100
Initialise multiple variable using Select statement :

SET can be used for only single variable where as SELECT can be used for multiple variable in a single statement

SELECT @Name=’Madhu’ , @Count=0 , @Address=’XYZ’

WHILE(@i<@Count)
BEGIN
SET @i=@i+1
PRINT @i
END

Oracle (PL/SQL) : Declaring Variables
In oracle, Variables can have any SQL datatype, such as CHAR, DATE, or NUMBER, or a PL/SQL-only datatype, such as BOOLEAN or PLS_INTEGER. For example, assume that you want to declare variables for part data, such as part_no to hold 6-digit numbers and in_stock to hold the Boolean value TRUE or FALSE. You declare these and related part variables as shown in Example 1. Note that there is a semi-colon (;) at the end of each line in the declaration section.

Example 1–1. Declaring Variables in PL/SQL
DECLARE
part_no NUMBER(6);
part_name VARCHAR2(20);

You can also declare nested tables, variable-size arrays (varrays for short), and records using the TABLE, VARRAY, and RECORD composite datatypes.

Assigning Values to a Variable
You can assign values to a variable in three ways. The first way uses the assignment operator (:=), a colon followed by an equal sign, as shown in Example 2. You place the variable to the left of the operator and an expression, including function calls, to the right. Note that you can assign a value to a variable when it is declared.

Example 1–2 Assigning Values to Variables With the Assignment Operator
DECLARE
wages NUMBER;
hours_worked NUMBER := 40;
emp_rec1 employees%ROWTYPE;
country := UPPER('Canada');


The second way to assign values to a variable is by selecting (or fetching) database values into it. In Example 1–3, 10% of an employee's salary is selected into the bonus variable. Now you can use the bonus variable in another computation or insert its value into a database table.

Example 1–3 Assigning Values to Variables by SELECTing INTO

DECLARE
bonus NUMBER(8,2);
emp_id NUMBER(6) := 100;
BEGIN
SELECT salary * 0.10 INTO bonus FROM employees
WHERE employee_id = emp_id;
END;

The third way to assign a value to a variable is by passing it as an OUT or IN OUT parameter to a subprogram, and then assigning the value inside the subprogram.

FAQ : How to find Product version in SQL Server and in Oracle

SQL Server

Select @@Version
Or
SET NOCOUNT ON;
SELECT
CONVERT( varchar, SERVERPROPERTY('Edition')) AS Edition
, CONVERT( varchar, SERVERPROPERTY('ProductVersion')) AS Version
, CONVERT( varchar, SERVERPROPERTY('ProductLevel')) AS Level


Oracle

select * From V$VERSION ;
Note : V$VERSION is the catalog view which provides all the information in Oracle
 
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