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Dbs101_flippedclass3

Posted Updated
By Tashi Penjor 1 min read

Topic : SQL set operation and null values

During the flip class I learn about the SQL set operation. SQL set operation is used to combine the two or more SQL statement. There are mainly 4 types of set operations namely union,union all,intersect,minus.

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Union in this operation when two table is joined the output will be combined into one result set. If there is duplicate value in the table it will be eliminated.

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Union All in this operation when two table is joined the output will is shown without eliminating the duplicates values in the table.

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Intersect in this operation the output shown will be those values in the rows that are common to both the table.

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Minus in this operation the output shown will be the column that are unique in the first query.

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During the flip class we discuss about the Null values in SQL. Null values are filed that are kept blank during the time of record creation because the information might not be available and also null value is different from every other null value. To test the null values it is not possible to use operator such as =,<,>, or <>. Instead we have to use IS NULL to test the empty values and IS NOT NULL to test the non-empty values. Moreover null values and zero value is completely different. On top of that null values has three interpretations. The unknown value,Value not available,Attribute not applicable.

To improve the next flip class I think we need to have database practical because we can learn more when doing the practical than the theory.

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