Difference Between NOLOCK and NOWAIT

NOWAIT will return error if the original table has (transaction) locked on it.
NOLOCK will read the data irrespective of the (transaction) lock on it.

In either case there can be incorrect data or unexpected result. There is no guarantee to get the appropriate data.

Here is the example of the how both provide different result on similar situation.

In this sample scenario we will create a table with a single row. We will open a transaction and delete a single row. We will now close the transaction and read the query with once with NOLOCK hint and once with NOWAIT hint. After the test we will rollback original transaction, which was deleting the record. As we will rollback transaction there will be no change in resultset however, we will get very different results in the case of NOLOCK and will get error in case of NOWAIT.

First Let us create a table:




Now let us open three different connections.

Run following command in the First Connection:




Run following command in the Second Connection:




Run following command in the Third Connection:




You can notice that result is as discussed earlier. There is no guarantee to get 100% correct result in either case. In the case of NOLOCK and will get error in case of NOWAIT. If you want to get the committed appropriate result, you should wait till the transaction lock on the original table is released and read the data.

SQL SERVER how to launch to Performance Monitor

There are three ways to launch  Performance Monitor.

1) Type “start perfmon” at the command prompt.

2) Go to Start | Programs | Administrative Tools | Performance Monitor.

3) Go to Start | Run | Perfmon.

Follow the images which explains how to use Perfmon and add different counters.

SQL SERVER Get Current Database Name

SELECT DB_NAME() AS DataBaseName

It will give the name of the database that currently running while running the query.

SQL PRIMARY KEY Constraint

 The PRIMARY KEY constraint uniquely identifies each record in a database table.
Primary keys must contain unique values.
A primary key column cannot contain NULL values.
Each table should have a primary key, and each table can have only ONE primary key.
SQL PRIMARY KEY Constraint on CREATE TABLE:
The following SQL creates a PRIMARY KEY on the "P_Id" column when the "Persons" table is created:
SQL Server / Oracle / MS Access:                                                                                   CREATE TABLE Persons
(
P_Id int NOT NULL PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
MySQL / SQL Server / Oracle / MS Access:                                                                  CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
)
MySQL:                                                                                                                              CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
PRIMARY KEY (P_Id)
)
SQL PRIMARY KEY Constraint on ALTER TABLE
MySQL / SQL Server / Oracle / MS Access:                                                                      ALTER TABLE Persons
ADD CONSTRAINT pk_PersonID PRIMARY KEY (P_Id)
To DROP a PRIMARY KEY Constraint
MySQL / SQL Server / Oracle / MS Access:                                                                            ALTER TABLE Persons
DROP CONSTRAINT pk_PersonID

SQL UNIQUE Constraint

The UNIQUE constraint uniquely identifies each record in a database table.
The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.
A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.
Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table. The following SQL creates a UNIQUE constraint on the "P_Id" column when the "Persons" table is created:
SQL Server / Oracle / MS Access:                                                                                  CREATE TABLE Persons
(
P_Id int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
MySQL / SQL Server / Oracle / MS Access:                                                                      CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
)
MySQL                                                                                                                              CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id)
)
SQL UNIQUE Constraint on ALTER TABLE
For single Col-    ALTER TABLE Persons ADD UNIQUE (P_Id)
For multi Col- ALTER TABLE Persons ADD CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
For Drop:
  MySQL- ALTER TABLE Persons DROP INDEX uc_PersonID
  SQL Server / Oracle / MS Access- ALTER TABLE Persons DROP CONSTRAINT uc_PersonID

SQL NOT NULL Constraint

The NOT NULL constraint enforces a column to NOT accept NULL values.
The NOT NULL constraint enforces a field to always contain a value. This means that you cannot insert a new record, or update a record without adding a value to this field.
The following SQL enforces the "P_Id" column and the "LastName" column to not accept NULL values:
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)

The SQL Constraints

Constraints are used to limit the type of data that can go into a table.
Constraints can be specified when a table is created (with the CREATE TABLE statement) or after the table is created (with the ALTER TABLE statement).
We will focus on the following constraints:
NOT NULL
UNIQUE
PRIMARY KEY
FOREIGN KEY
CHECK
DEFAULT

The ALTER TABLE Statement

The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.
Syntax: 
To add-  ALTER TABLE <table_name> ADD <column_name>  datatype
To delete a col-  ALTER TABLE table_name DROP COLUMN column_name
To chng datatyp of col-  ALTER TABLE table_name ALTER COLUMN column_name datatype

The CREATE TABLE Statement

The CREATE TABLE statement is used to create a table in a database.
Syntax:
CREATE TABLE <table_name>
(
<column_name1> <data_type>,
<column_name2> <data_type>,
<column_name3> <data_type>,
....
)
Example:
 Now we want to create a table called "Persons" that contains five columns: P_Id, LastName, FirstName, Address, and City.
Query:
CREATE TABLE Persons
(
P_Id int,
LastName varchar(255),
FirstName varchar(255),
Address varchar(255),
City varchar(255)
)
Output:
The empty "Persons" table will now look like this:

P_Id    LastName    FirstName    Address    City      
                   

The CREATE DATABASE Statement

The CREATE DATABASE statement is used to create a database.
Syntax:           CREATE DATABASE <database_name>
Example:         CREATE DATABASE my_db

Parts ofSQL

SQL can be divided into two parts: The Data Manipulation Language (DML) and the Data Definition Language (DDL).
The query and update commands form the DML part of SQL:
SELECT - extracts data from a database
UPDATE - updates data in a database
DELETE - deletes data from a database
INSERT INTO - inserts new data into a database
The DDL part of SQL permits database tables to be created or deleted. It also defines indexes (keys), specifies links between tables, and imposes constraints between tables. The most important DDL statements in SQL are:

CREATE DATABASE - creates a new database
ALTER DATABASE - modifies a database
CREATE TABLE - creates a new table
ALTER TABLE - modifies a table
DROP TABLE - deletes a table
CREATE INDEX - creates an index (search key)
DROP INDEX - deletes an index

can i put Semicolon after SQL Statements?

Some database systems require a semicolon at the end of each SQL statement.
Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server.
We are using MS Access and SQL Server 2000 and we do not have to put a semicolon after each SQL statement, but like MySQL database programs force you to use it.

What Can SQL do?

SQL can execute queries against a database
SQL can retrieve data from a database
SQL can insert records in a database
SQL can update records in a database
SQL can delete records from a database
SQL can create new databases
SQL can create new tables in a database
SQL can create stored procedures in a database
SQL can create views in a database
SQL can set permissions on tables, procedures, and views
NOTE:   SQL is not case sensitive

What is SQL?

SQL stands for Structured Query Language
SQL lets you access and manipulate databases
SQL is an ANSI (American National Standards Institute) standard