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SQL Questions 3
Posted Date: 19 Sep 2008 Resource Type: Articles Category: Databases
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Posted By: saravanan Member Level: Gold
Rating:  Points: 7
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What is Aggregate Functions?
Aggregate functions perform a calculation on a set of values and return a single value. Aggregate functions ignore NULL values except COUNT function. HAVING clause is used, along with GROUP BY, for filtering query using aggregate values.
Following functions are aggregate functions.
AVG, MIN, CHECKSUM_AGG, SUM, COUNT, STDEV, COUNT_BIG, STDEVP, GROUPING, VAR, MAX, VARP
What do you mean by Table Sample?
TABLESAMPLE allows you to extract a sampling of rows from a table in the FROM clause. The rows retrieved are random and they are not in any order. This sampling can be based on a percentage of number of rows. You can use TABLESAMPLE when only a sampling of rows is necessary for the application instead of a full result set.
What is Row_Number()?
ROW_NUMBER() returns a column as an expression that contains the row’s number within the result set. This is only a number used in the context of the result set, if the result changes, the ROW_NUMBER() will change.
What are Ranking Functions?
Ranking functions return a ranking value for each row in a partition. All the ranking functions are non-deterministic. Different Ranking functions are:
ROW_NUMBER () OVER ([] )
Returns the sequential number of a row within a partition of a result set, starting at 1 for the first row in each partition.
RANK () OVER ([] )
Returns the rank of each row within the partition of a result set.
DENSE_RANK () OVER ([] )
Returns the rank of rows within the partition of a result set, without any gaps in the ranking.
What is the difference between UNION and UNION ALL?
UNION
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type. With UNION, only distinct values are selected.
UNION ALL
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.
The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all rows from all tables fitting your query specifics and combines them into a table.
What is B-Tree?
The database server uses a B-tree structure to organize index information. B-Tree generally has following types of index pages or nodes:
* root node: A root node contains node pointers to branch nodes which can be only one.
* branch nodes: A branch node contains pointers to leaf nodes or other branch nodes which can be two or more.
* leaf nodes: A leaf node contains index items and horizontal pointers to other leaf nodes which can be many.
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Responses
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| Author: balamurugan 19 Sep 2008 | Member Level: Silver Points : 1 | ya ur right
Following functions are aggregate functions.
AVG, MIN, CHECKSUM_AGG, SUM, COUNT, STDEV, COUNT_BIG, STDEVP, GROUPING, VAR, MAX, VARP
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