Archive for September 15, 2014

Difference between Temporary Table and Table Variable, which one is better in performance? – MSDN TSQL forum

September 15, 2014 Leave a comment

–> Question:

Anyone could you explain What is difference between Temp Table (#, ##) and Table Variable (DECLARE @V TABLE (EMP_ID INT)) ?

Which one is recommended to use for better performance?

Also is it possible to create CLUSTER and NONCLUSTER Index on Table Variables?

In my case: 1-2 days transnational data are more than 3-4 Millions. I tried using both # and table variable and found table variable is faster.

Is that Table variable using Memory or Disk space?

–> My Answer:

Check this link to see differences b/w Temp Table & Table Variable.

TempTables & TableVariables both use memory & tempDB in similar manner, check this blog post.

Performance wise if you are dealing with millions of records then Temp Table is ideal, as you can create explicit indexes on top of them. But if there are less records then Table Variables are good suited.

On Tables Variable explicit index are not allowed, if you define a PK column, then a Clustered Index will be created automatically.

Ref Link.

DB Basics – What are Candidate, Primary, Composite & Super Keys and Difference between them?

September 15, 2014 5 comments

Let’s take an example of an Employee table:

    DOB, -- Date of birth
    DOJ, -- Date of joining
    SSN, -- Social Security Number
    DeptID, -- Department ID
    MgrID -- Manager ID

1. Candidate Key: is the attribute/column or a set of attributes/columns in a relation/table that qualifies for uniqueness of each tuple/row. A relation/table can have one or more than one Candidate Keys. A Candidate key is also known as a minimal Super key.

Here in Employee table columns EmployeeID & SSN individually can maintain uniqueness in a table, thus are eligible for Candidate keys. The columns EmployeeName + DOB combined can also make up a Candidate Key, but there is a narrow chance that 2 Employees with same name can be born in same day.

2. Primary Key: is the Candidate key attribute/column that is most suited to maintain uniqueness in a table at the tuple/row level. More about PK.

Here in Employee table you can choose either EmployeeID or SSN column for a PK, EmployeeID is preferable choice because SSN is a secure (PII) value.

3. Alternate Key: are the other Candidate key attribute/columns that you didn’t choose as Primary key column.

Like if you choose EmployeeID as a PK then SSN would be the Alternate key.

4. Super Key: is a superset of Candidate key. If you add any other attribute/column to a Candidate Key then it become a Super Key.

Like EmployeeID + EmployeeName is a Super Key.

5. Composite Key: If a table do have a single column that qualifies for a Candidate key, then you have to select 2 or more columns to make a row unique.

Like if there is no EmployeeID or SSN columns in Employee table, then you can make EmployeeName + DOB as a Composite Primary Key. But still there can be a narrow chance of duplicate rows.

–> Check the video:

DBMS Keys Types