UPDATE/ADD/ALTER Column and Table in SQL


The following SQL statement updates the first customer (CustomerID = 1) with a new contact person and a new city.


UPDATE Customers
SET ContactName = 'Alfred Schmidt', City= 'Frankfurt'
WHERE CustomerID = 1;

UPDATE Multiple Records

It is the WHERE clause that determines how many records that will be updated.

The following SQL statement will update the contactname to “Juan” for all records where country is “Mexico”:


UPDATE Customers
SET ContactName='Juan'
WHERE Country='Mexico';

Update Warning!

Be careful when updating records. If you omit the WHERE clause, ALL records will be updated!


UPDATE Customers
SET ContactName='Juan';


To add a column in a table, use the following syntax:

ALTER TABLE table_name
ADD column_name datatype;


To change the data type of a column in a table, use the following syntax:

SQL Server / MS Access:

ALTER TABLE table_name
ALTER COLUMN column_name datatype;


WHERE CustomerName='Alfreds Futterkiste';

DELETE FROM table_name
WHERE condition;

Delete All Records

DELETE * FROM table_name;


To delete a column in a table, use the following syntax (notice that some database systems don’t allow deleting a column):

ALTER TABLE table_name
DROP COLUMN column_name;

SQL Inteview Questions

SET @first = 1
SET @step = 1
SET @last = 1000

WHILE(@first <= @last) BEGIN INSERT INTO TEST_NUMBER VALUES(@first) SET @first += @step


SELECT TOP (1000) [IncrNum]

-- SELECT [IncrNum] FROM 1 to 1000
WHERE [IncrNum] <= 1000

WHERE [IncrNum] <= 1000
AND ([IncrNum] % 2 <> 0)

SQL – Queries Tuning and Optimization Techniques

In SQL, it is very difficult to write complex SQL queries involving joins across many (at least 3-4) tables and involving several nested conditions because a SQL statement, once it reaches a certain level of complexity, is basically a little program in and of itself.

A database index is a data structure that improves the speed of operations on a database table. Indexes can be created using one or more columns of a database table, providing the basis for both rapid random lookups and efficient access of ordered records. Indexing is incredibly important when working with large tables, however, occasionally smaller tables should be indexed, if they are expected to grow.

Try to consistently indent and don’t be afraid to use multiple lines. You don’t have to write it all at once. Complex queries can sometimes just be a collection of simple queries. You need to follow some basic guidelines and Take the time to think these through such as-

List all of the columns that are to be returned

  1. List all of the columns that are used in the WHERE clause
  2. List all of the columns used in the JOINs (if applicable)
  3. List all the tables used in JOINs (if applicable)
  4. Get the correct records selected first
  5. Save the complex calculations for last
  6. If you do use a Common Table Expression (CTE), be aware that the query only persists until the next query is run, so in some cases where you are using the CTE in multiple queries, it might be better for performance to use a temp table.


Once you have the above information organized into this easy-to-comprehend form, it is much easier to identify those columns that could potentially make use of indexes when executed.

It makes a big difference to really understand how the data is combined, selected, filtered, and output. Here, query Optimization tricks come into the picture to increase the performance of the program or software. There are a lot of guideline points to tune your query which do work as the boost of the query performance.  These guideline points are mentioned below:

  1. SET NOCOUNT ON at the beginning of each stored procedure you write. This statement should be included in every stored procedure, trigger, etc. that you write.
  2. The SQL query becomes faster if you use the actual columns names in SELECT statement instead of than ‘*’.
  3. HAVING clause is used to filter the rows after all the rows are selected if you are using aggregation functions. It is just like a filter. Do not use HAVING clause for any other purposes.
  4. It is the best practice to avoid sub queries in your SQL statement and try to minimize the number of subquery block in your query if possible.
  5. Use operator EXISTS, IN and table joins appropriately in your query. The reason is- Usually IN has the slowest performance
  6. IN is efficient when most of the filter criteria are in the sub-query.
  7. EXISTS is efficient when most of the filter criteria is in the main query.
  8. Use EXISTS instead of DISTINCT when using joins which involves tables having one-to-many relationship.
  9. Be careful while using conditions in WHERE clause.
  10. To write queries which provide efficient performance follow the general SQL standard rules.
  11. Use single case for all SQL verbs
  12. Begin all SQL verbs on a new line
  13. Separate all words with a single space
  14. Right or left aligning verbs within the initial SQL verb
  15. Indexes have the advantages as well as disadvantages as given below-
  16. Do not automatically add indexes on a table because it seems like the right thing to do. Only add indexes if you know that they will be used by the queries run against the table.
  17. Indexes should be measured on all columns that are frequently used in WHERE, ORDER BY, GROUP BY, TOP and DISTINCT clauses.
  18. Do not add more indexes on your OLTP tables to minimize the overhead that occurs with indexes during data modifications.
  19. Drop all those indexes that are not used by the Query Optimizer, generally.
  20. If possible, try to create indexes on columns that have integer values instead of characters. Integer values use less overhead than character values.
  21. To provide up-to-date statistics, the query optimizer needs to make smart query optimization decisions. You will generally want to leave the “Auto Update Statistics” database option on. This helps to ensure that the optimizer statistics are valid, ensuring that queries are properly optimized when they are run.
  22. If you want to boost the performance of a query that includes an AND operator in the WHERE clause, consider the following:
  23. Of the search criteria in the WHERE clause, at least one of them should be based on a highly selective column that has an index.
  24. If at least one of the search criteria in the WHERE clause is not highly selective, consider adding indexes to all of the columns referenced in the WHERE clause.
  25. If none of the columns in the WHERE clause are selective enough to use an index on their own, consider creating a covering index for this query.
  26. queries that include either the DISTINCT or the GROUP BY clauses can be optimized by including appropriate indexes. Any of the following indexing strategies can be used:
  27. Include a covering, non-clustered index (covering the appropriate columns) of the DISTINCT or the GROUP BY clauses.
  28. Include a clustered index on the columns in the GROUP BY clause.
  29. Include a clustered index on the columns found in the SELECT clause.
  30. Adding appropriate indexes to queries that include DISTINCT or GROUP BY is most important for those queries that run often.
  31. When you need to execute a string of Transact-SQL, you should use the sp_executesql stored procedure instead of the EXECUTE statement.
  32. When calling a stored procedure from your application, it is important that you call it using its qualified name.
  33. Use stored procedures instead of views because they offer better performance and don’t include code, variable or parameters that don’t do anything.
  34. If possible, avoid using SQL Server cursors. They generally use a lot of SQL Server resources and reduce the performance and scalability of your applications.
  35. Instead of using temporary tables, consider using a derived table instead. A derived table is the result of using a SELECT statement in the FROM clause of an existing SELECT statement. By using derived tables instead of temporary tables, you can reduce I/O and often boost your application’s performance.
  36. Don’t use the NVARCHAR or NCHAR data types unless you need to store 16-bit character (Unicode) data. They take up twice as much space as VARCHAR or CHAR data types, increasing server I/O and wasting unnecessary space in your buffer cache.
  37. If you use the CONVERT function to convert a value to a variable length data type such as VARCHAR, always specify the length of the variable data type. If you do not, SQL Server assumes a default length of 30.
  38. If you are creating a column that you know will be subject to many sorts, consider making the column integer-based and not character-based. This is because SQL Server can sort integer data much faster than character data.
  39. Don’t use ORDER BY in your SELECT statements unless you really need to, as it adds a lot of extra overhead. For example, perhaps it may be more efficient to sort the data at the client than at the server.
  40. Don’t return more data (both rows and columns) from SQL Server than you need to the client or middle-tier and then further reduce the data to the data you really need at the client or middle-tier. This wastes SQL Server resources and network bandwidth.
To tune our SQL queries, understanding our database does play the most important role. In SQL, typically each table column has an associated data type. Text, Integer, Varchar, Date, and more, are typically available types for developers to choose from. When writing SQL statements, make sure you choose the proper data type for the column. Sometimes it’s easier to break up subgroups into their own select statement. To write a query, we need to know about the actual need for the query and scope of the query also.




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Quick Tricls: Column and block text selection using SSMS

There are several ways to select text as shown below, including the ability to select and edit columns.

Using SHIFT to Select Text

It is well known that using the SHIFT key you can perform normal text selection in SSMS.

If you put your cursor to the left of “dbo.DimEmployee” and hold the SHIFT key and then put your cursor at the end of “dbo.DimReseller” it will select the first three lines of code as shown below.

ssms shift select

Using SHIFT+ALT to Select Columns

If you would like to select columns or blocks then Microsoft SQL Server offers a solution for you. You can use the key shortcut SHIFT+ALT as described in the following steps. Please note that this feature works using SSMS for SQL Server 2008 and up.

Place your cursor to the left of “dbo.DimEmployee”, press SHIFT+ALT then click at the end of “dbo” in “dbo.DimProductCategory”. This will select columns or blocks in SQL Server Management Studio as shown below.

ssms shift alt select

Using SHIFT+ALT to Select Columns and Insert Text

In SSMS for SQL Server 2012 and up, you can also use SHIFT+ALT to insert text in this block mode.

First place the cursor in the first row where you would like to insert the text (to the left dbo.DimEmployee in our example). Press SHIFT+ALT and click in the last line where you would like to append this text (left of dbo.DimProductCategory). Now type “SELECT * FROM ” and this text will be inserted for each line as shown below.

If you would like to select columns or blocks then Microsoft SQL Server offers a solution for you

Using CTRL+SHIFT+END to Select Text

If you want to select all text from a starting point to the end you can use CTRL+SHIFT+END.

Put your cursor at the beginning point and press CTRL+SHIFT+END to select all text from that point to the end of the text as shown below.

ssms ctrl shift end to select

Using CTRL+SHIFT+HOME to Select Text

If you want to select all text from a starting point to the beginning you can use CTRL+SHIFT+HOME.

Put your cursor at the beginning point and press CTRL+SHIFT+HOME to select all text from that point to the beginning of the text as shown below.

ssms ctrl shift home select

Using CTRL+A to Select All Text

If you want to select all text you can use CTRL+A.

Just press CTRL+A anywhere in the query editor and this will select all text as shown below.

ssms ctrl a to select all text


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SQL Server Data Types you Must Know

Why data types are important

  1. The data is stored in the database in a consistent and known format.
  2. Knowing the data type allows you to know which calculations and formulations you can use on the column.
  3. Data types affect storage. Some values take up more space when stored in one data type versus another.  Take our age tables above for example.
  4. Data types affect performance. The less time the database has to infer values or convert them the better.  “Is December, 32, 2015 a date?”

Commonly used SQL Server Data Types

There are over thirty different data types you can choose from when defining columns.  Many of these are set up for very specific jobs such as storing images, and others more suitable to general use.

Here is the data types you’ll most frequently encounter in your everyday use of SQL.  These are:

  • INT
  • BIT

INT – Integer Data Type

The integer data type is used to store whole numbers.  Examples include -23, 0, 5, and 10045.  Whole numbers don’t include decimal places.  Since SQL server uses a number of computer words to represent an integer there are maximum and minimum values which it can represent.  An INT datatype can store a value from -2,147,483,648 to 2,147,483,647.

Practical uses of the INT data type include using it to count values, store a person’s age, or use as an ID key to a table.

But INT wouldn’t be so good to keep track of a terabyte hard drive address space, as the INT data type only goes to 2 billion and we would need to track into the trillions.  For this you could use BIGINT.

The INT data type can be used in calculations.  Since DaysToManufacture is defined as INT we can easily calculate hours by multiplying it by 24:

       DaysToManufacture * 24 as HoursToManufacture
FROM   Production.Product

Here you can see the results

Use of INT to perform calculations.

There are many operations and functions you can use with integers which we’ll cover once we dig into functions.

VARCHAR and NVARCHAR – Text Values

Both VARCHAR and NVARCHAR are used to store variable length text values.  “VARCHAR” stands for variable length character.

The number of characters to store in a VARCHAR or NVARCHAR are defined within the column.   For instance as you can see in the following column definition from the object explorer, the product name is defined to hold fifty characters.

VARCHAR definition shown in SQL Server Management Studio

What makes VARCHAR popular is that values less than fifty characters take less space.  Only enough space to hold the value is allocated.  This differs from the CHAR data type which always allocates the specified length, regardless of the length of the actual data stored.

The VARCHAR datatype can typically store a maximum of 8,000 characters.  The NVARCHAR datatype is used to store Unicode text.  Since UNICODE characters occupy twice the space, NVARCHAR columns can store a maximum of 4,000 characters.

The advantage NVARCHAR has over VARCHAR is it can store Unicode characters.  This makes it handy to store extended character sets like those used for languages such as Kanji.

If your database was designed prior to SQL 2008 you’ll most likely encounter VARCHAR; however, more modern databases or those global in nature tend to use NVARCHAR.

DATETIME – Date and Time

The DATETIME data type is used to store the date and time.  An example of a DATATIME value is

1968-10-23 1:45:37.123

This is the value for October 23rd, 1968 at 1:45 AM.  Actually the time is more precise than that.  The time is really 45 minutes, 37.123 seconds.

In many cases you just need to store the date.  In these cases, the time component is zeroed out.  Thus, November 5th, 1972 is

1972-11-05 00:00:00.000

A DATETIME can store dates from January 1, 1753, through December 31, 9999.  This makes the DATETIME good for recording dates in today’s world, but not so much in William Shakespeare’s.

As you get more familiar with the various SQL built-in functions you’ll be able to manipulate the data.  To give you a glimpse, we’ll use the YEAR function to count employees hired each year.  When given a DATETIME value, the YEAR function return the year.

The query we’ll use is

SELECT   YEAR(HireDate),
FROM     HumanResources.Employee

And here are the results

Use YEAR on DATETIME data type

The benefit is the DATETIME type ensures the values are valid dates.  Once this is assured, we’re able to use a slew of functions to calculate the number of days between dates, the month of a date and so on.

We’ll explore these various functions in detail in another blog article.

DECIMAL and FLOAT – Decimal Points

As you may have guessed DECIMAL and FLOAT datatypes are used to work with decimal values such as 10.3.

I lumped DECIMAL and FLOAT into the same category, since they both can handle values with decimal points; however, they both do so differently:

If you need precise values, such as when working with financial or accounting data, then use DECIMAL.  The reason is the DECIMAL datatype allows you to define the number of decimal points to maintain.


DECIMAL data types are defined by precision and scale.  The precision determine the number of total digits to store; whereas, scale determine the number of digits to the right of the decimal point.

A DECIMAL datatype is specified as DECIMAL(precision,scale).

A DECIMAL datatype can be no more than 38 digits.  The precision and scale must adhere to the following relation

0 <= scale <= precision <= 38 digits

In the Production.Product table, the weight column’s datatype is defined as DECIMAL(8,2).  The first digit is the precision, the second the scale.

Weight is defined to have eight total digits, two of them to the right of the decimal place.  We’ll the following sample query to illustrate how this data type.

FROM     Production.Product
WHERE    Weight BETWEEN 29.00 and 189.00

The results follow:

Using DECIMAL data type to display results


Where DECIMAL datatypes are great for exact numbers, FLOATs are really good for long numeric values.  Though a DECIMAL value can have 38 digits total, in many engineering and scientific application this is inadequate.  For scientific applications where extreme numeric values are encountered, FLOAT rises to the top!

FLOATS have a range from – 1.79E+308 to 1.79E+308.  That means the largest value can be 179 followed by 306 zeros (large indeed!).

Because of the way float data is stored in the computer (see IEEE 754 floating point specification) the number stored is an extremely close approximation.  For many application this is good enough.

Because of the approximate behavior, avoid using <> and = operators in the WHERE clause.  Many a DBA has been burned by the statement.

WHERE mass = 2.5

Their expectation are dashed when mass is supposed to equal 2.5, but really, in the computer it is stored as 2.499999999999999; therefore, not equal to 2.500000000000000!

That is the nature of floating points and computers.  You and I see 2.499999999999999 and think for practical purposes it is 2.5, but to the computer, were off just a bit.  J

BIT – Boolean or Yes/No values

There’s times when you just need to store whether something “is” or “is not.”  For instance, whether an employee is active.  It is in these cases that the BIT datatype comes to its own.  This data type be one of three states: 1, 0, or NULL.

The value of 1 signifies TRUE and 0 FALSE.

In this query we’re listing all salaried position job titles

FROM   HumanResources.Employee
WHERE  SalariedFlag = 1

Here are the results

Using the BIT data type in Searches

We could have also use ‘True’ instead of 1.  Here is the same example using ‘True’

FROM   HumanResources.Employee
WHERE  SalariedFlag = 'True'

And the opposite using ‘False’

FROM   HumanResources.Employee
WHERE  SalariedFlag = 'False'

I tend to stick with 1 and 0, since it is easier to type, but if you’re going for readability, then ‘True’ and ‘False’ are good options.

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The Coding Naming Convention

How to make your colleagues’ lives easier while reviewing your codes?

– Please just follow the naming convention.

For me personally following the Naming Convention is a basic manner and a friendly way to treat someone who will read your code. It reflects that you have attention to detail and logical thinking. It’s also one of the criteria to determine your coding quality.

My training manager had said something about this, ‘I will see whether you are a professional developer even your code doesn’t even work’.

Some bad examples and some tips:

  • Do not give full and descriptive names. eg. FController or FinController instead of FinanceController. Avoid using unknown acronyms is necessary.
  • Pascal Casing is important. E.g. ProductCategory, ChildProgram
  • Do not provide comments on the code which may look confusing. Provide comments on your assumptions is effective.
  • Use new lines to indicate a new logical group.
  • Always Indent the code properly
  • Use Ctrl+K+D to auto-indent your code in Visual Studio.
  • Retain consistency in styles.

Install AdventureWorks2014 and AdventureWorksDW2014 Step by Step

Before installation, please download 2 sample databases from below,



Open SQL Server Management Studio, Right click on “Databases”, select “Restore Database”,

Select “Device” and click button on the right,

Click “Add” button,

Select AdventureWorks2014.bak, click “OK” button.

Click “OK” button

Click “OK” button.

AdventureWorks2014 has been installed.

Please refer above steps to install AdventureWorksDW2014, it’s exactly the same steps.




SQL Exercise 01

All exercises would be based on the dataset Book:


Write SQL statements for following queries.

  1. List the first and last names of authors from Auckland. You must match all possible cases of the word Auckland.
  2. Use a SELECT statement to display the following result using the inventory and transaction type tables
  3. Show a list of all book titles and their types. For books without a type show ‘UKN’.
  4. Complete the query below to show the revised salary of staff based on their role. Use the following as a guide:                                                                                                         – Branch Manager: 90% of original salary                                                                               – Sales Person: no change                                                                                                   – —     – Office admin: 115% of original salary                                                                                   – Hint: Check out how to use the CASE expression from SQLite documentation
  5. Select the title from the book table, and the author number, publisher code, and edition from the writing table. Don’t use a join condition – i.e. no WHERE clause. This will display the result of a Cartesian product.
  6. Write a query to list the title from the book table, and the author number, publisher code, and edition from the writing table.
  7. Write a query to list the title and book code from the book table, and the book code, author number, publisher code, and edition from the writing table.
  8. Write a query to select the book title, publisher name, and edition for all writings.
  9. For each bookprice, show the book code, price, and book grade. Book grades are defined in the bookgrade table.
  10.  For each bookprice, show the book code, book title, price, and book grade.
  11. Create a query to list book inventory transactions. Show the book title, transaction number, date, and quantity. Show all book titles even if they don’t have any transactions.
  12. Create a query to list book inventory transactions involving quantities less than 100. Show the book title, transaction number, date, and quantity. Treat all book titles without any transactions as having a quantity of 0, which means they should be included in the query as well.