Getting started with Microsoft SQL Server

Examples

• 20

  Basic DML Operations in MS SQL Server

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  20

  DML operations such as INSERT, UPDATE and DELETE:

  -- Create a table variable, which can be treated like a normal table
  
  DECLARE @HelloWorld TABLE (
      ID INT IDENTITY,
      Description VARCHAR(1000)
  )
  
  
  -- DML Operation - Insert, Inserting a row into the table
  INSERT INTO @HelloWorld (Description) VALUES ('Hello, World!')
  
  
  -- Displaying the table 
  SELECT * FROM @HelloWorld  
  
  -- Select a specific column from table
  SELECT Description FROM @HelloWorld
  
  
  -- Display number of records in the table
  SELECT Count(*) FROM @HelloWorld
  
  
  -- DML Operation - Update, Updating a specific row in the table
  UPDATE @HelloWorld SET Description = 'Hello World!' WHERE ID = 1
  
  
  -- Selecting the table. See how the DESCRIPTION changed after the UPDATE?
  SELECT * FROM @HelloWorld
  
  
  -- DML Operation - Delete, Deleting a row from the table
  DELETE FROM @HelloWorld WHERE ID = 1
  
  
  -- Selecting the table 
  SELECT * FROM @HelloWorld
  

  In this script we're creating a table variable, which resides in memory only, not in a database, to demonstrate some basic queries.

  The @ symbol in the example above needs to be used as a prefix for all variables you declare in T-SQL. Its purpose is to distinguish between a variable and a table, e.g.

  Use Northwind;
  go
  SELECT TOP 10 * FROM Customers 
  ORDER BY CompanyName
  

  will select the first 10 records of the Customer table, ordered by the column CompanyName from the database Northwind:

  

  Note that Use Northwind; changes the default database for all subsequent queries. You can still reference the database by using the fully qualified syntax:

  SELECT TOP 10 * FROM Northwind.dbo.Customers 
  ORDER BY CompanyName
  
  SELECT TOP 10 * FROM Pubs.dbo.Authors
  ORDER BY City
  

  This is useful, if you're querying data from different databases. Note that dbo, specified "in between" is called a schema and needs to be specified always when using the fully qualified syntax.

  If the database table contains columns which are named like reserved words, e.g. Date, you need to enclose the column name in brackets, like so:

  -- descending order
  Select top 10 [Date] from dbo.MyLogTable
  order by [Date] desc 
  

  The same applies if the column name contains spaces in its name (which is not recommended).

  edited yesterday

  

  

  

  

  

  +9
• 3

  SELECT all rows and columns from a table

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  3

  Syntax:

  SELECT *
  FROM table_name
  

  Using the asterisk operator * serves as a shortcut for selecting all the columns in the table. All rows will also be selected because this SELECT statement does not have a WHERE clause, to specify any filtering criteria.

  This would also work the same way if you added an alias to the table, for instance e in this case:

  SELECT *
  FROM Employees AS e
  

  Or if you wanted to select all from a specific table you can use the alias + " .* ":

  SELECT e.*, d.DepartmentName
  FROM Employees AS e
      INNER JOIN Department AS d 
          ON e.DepartmentID = d.DepartmentID
  

  Database objects may also be accessed using fully qualified names:

  SELECT * FROM [server_name].[database_name].[schema_name].[table_name]
  

  This is not necessarily recommended, as changing the server and/or database names would cause the queries using fully-qualified names to no longer execute due to invalid object names.

  Note that the fields before table_name can be omitted in many cases if the queries are executed on a single server, database and schema, respectively. However, it is common for a database to have multiple schema, and in these cases the schema name should not be omitted when possible.

  edited Jul 27 at 21:07

  

  

  

  

  

  +1
• 3

  UPDATE Specific Row

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  3

  UPDATE HelloWorlds
  SET HelloWorld = 'HELLO WORLD!!!'
  WHERE Id = 5
  

  In this case when you use UPDATE like this, we will update value of column HelloWorld for record with Id = 5, after you execute this statement new value for record with Id = 5 will be: "HELLO WORLD!!!"

  edited Jul 27 at 6:22

  

  
• 2

  Comments in code

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  2

  Transact-SQL supports two forms of comment writing. Comments are ignored by the database engine, and are meant for people to read.

  Comments are preceded by -- and are ignored until a new line is encountered:

  -- This is a comment
  SELECT *
  FROM MyTable -- This is another comment
  WHERE Id = 1;
  

  Slash star comments begin with /* and end with */. All text between those delimiters is considered as a comment block.

  /* This is
  a multi-line
  comment block. */
  SELECT Id = 1, [Message] = 'First row'
  UNION ALL
  SELECT 2, 'Second row'
  /* This is a one liner */
  SELECT 'More';
  

  Slash star comments have the advantage of keeping the comment usable if the SQL Statement loses new line characters. This can happen when SQL is captured during troubleshooting.

  Slash star comments can be nested and a starting /* inside a slash star comment needs to be ended with a */ to be valid. The following code will result in an error

  /*
  SELECT *
  FROM CommentTable
  WHERE Comment = '/*'
  */
  

  The slash star even though inside the quote is considered as the start of a comment. Hence it needs to be ended with another closing star slash. The correct way would be

  /*
  SELECT *
  FROM CommentTable
  WHERE Comment = '/*'
  */  */
  

  edited Aug 3 at 17:48

  

  

  

  
• 2

  PRINT

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  2

  Display a message to the output console. Using SQL Server Management Studio, this will be displayed in the Messages tab, rather than the Results tab:

  PRINT 'Hello World!';
  

  edited Jul 27 at 21:07

  

  

  
• 2

  Select rows that match a condition

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  2

  Generally, the syntax is:

  SELECT <column names>
  FROM <table name>
  WHERE <condition>
  

  For example:

  SELECT FirstName, Age
  FROM Users
  WHERE LastName = 'Smith'
  

  Conditions can be complex:

  SELECT FirstName, Age
  FROM Users
  WHERE LastName = 'Smith' AND (City = 'New York' OR City = 'Los Angeles')
  

  created Jul 21 at 15:19

  
• 1

  DELETE All Rows

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  1

  DELETE
  FROM Helloworlds
  

  This will delete all the data from the table. The table will hold no data after you run this code.

  edited Aug 1 at 18:57

  

  

  
• 1

  Retrieve Basic Server Information

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  1

  SELECT @@VERSION
  

  Returns the version of MS SQL Server running on the instance.

  SELECT @@SERVERNAME
  

  Returns the name of the MS SQL Server instance

  SELECT @@SERVICENAME
  

  Returns the name of the Windows service MS SQL Server is running as.

  created Jul 29 at 1:04

  
• 1

  UPDATE All Rows

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  1

  A simple form of updating is incrementing all the values in a given field of the table. In order to do so, we need to define the field and the increment value

  The following is an example that increments the Score field by 1 (in all rows):

  UPDATE Scores
  SET score=score+1  
  

  This can be dangerous since you can corrupt your data if you make accidently mistake an UPDATE for a specific Row with an UPDATE for All rows in the table.

  edited Aug 3 at 17:49

  

  

  
• 0

  Create new table and insert records from old table

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  0

  SELECT * INTO NewTable FROM OldTable
  

  Creates a new table with structure of old table and inserts all rows into the new table.

  created Aug 3 at 17:48

  
• 0

  Getting Table Row Count

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  0

  The following example can be used to find the total row count for a specific table in a database if table_name is replaced by the the table you wish to query:

  SELECT COUNT(*) AS [TotalRowCount] FROM table_name;
  

  It is also possible to get the row count for all tables by joining back to the table's partition based off the tables' HEAP (index_id = 0) or cluster clustered index (index_id = 1) using the following script:

  SELECT  [Tables].name                AS [TableName],
          SUM( [Partitions].[rows] )    AS [TotalRowCount]
  FROM    sys.tables AS [Tables]
  JOIN    sys.partitions AS [Partitions]
      ON  [Tables].[object_id]    =    [Partitions].[object_id]
      AND [Partitions].index_id IN ( 0, 1 )
  --WHERE    [Tables].name = N'table name' /* uncomment to look for a specific table */
  GROUP BY    [Tables].name;
  

  This is possible as every table is essentially a single partition table, unless extra partitions are added to it. This script also has the benefit of not interfering with read/write operations to the tables rows'.

  created Aug 3 at 17:49

  
• 0

  TRUNCATE TABLE

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  0

  TRUNCATE TABLE Helloworlds 
  

  This code will delete all the data from the table helloworlds. Truncate table is almost similar to Delete from Table code. The difference is that you can not use where clauses with Truncate. Truncate table is considered better than delete because it uses less transaction log spaces.

  Note that if an identity column exists, it is reset to the initial seed value. This can lead inconsistency if the identity columns is used as a foreign key in another table.

  edited Aug 3 at 5:27

  

  

  
• 0

  Using Transactions to change data safely

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  0

  Whenever you change data, in a Data Manipulation Language(DML) command, you can wrap your changes in a transaction. DML includes UPDATE, TRUNCATE, INSERT and DELETE. One of the ways that you can make sure that you're changing the right data would be to use a transaction.

  DML changes will take a lock on the rows affected. When you begin a transaction, you must end the transaction or all objects being changed in the DML will remain locked by whoever began the transaction. You can end your transaction with either ROLLBACK or COMMIT. ROLLBACK returns everything within the transaction to its original state. COMMIT places the data into a final state where you cannot undo your changes without another DML statement.

  Example:

  --Create a test table
  
  USE [your database]
  GO
  CREATE TABLE test_transaction (column_1 varchar(10))
  GO
  
  INSERT INTO 
   dbo.test_transaction
          ( column_1 )
  VALUES
          ( 'a' )
  
  BEGIN TRANSACTION --This is the beginning of your transaction
  
  UPDATE dbo.test_transaction
  SET column_1 = 'B'
  WHERE column_1 = 'A'
  
  SELECT * FROM dbo.test_transaction  --This line will show you what changed
  
  ROLLBACK TRANSACTION  --Rollback will undo your changes
             --Alternatively, use COMMIT to save your results
  
  SELECT * FROM dbo.test_transaction   --View the table after your changes have been run
  
  DROP TABLE dbo.test_transaction
  

  created Jul 29 at 1:06