Why is there a performance difference between two similar queries where one uses union and the other uses union all?

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What are the kinds of subquery?

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Explain basic SQL queries with SELECT from where Order By, Group By-Having?

3 Answers   TetraSoft, Wipro,

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You are designing a database for your human resources department in the employee table, there is a field for social security number, which cannot contain null values if no value is given, you want a value of unknown to be inserted in this field what is the best approach?

0 Answers  

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What is difference between oltp and olap?

0 Answers  

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Explain Active/Active and Active/Passive cluster configurations

2 Answers  

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you added a row to a view, but the row is not shown on the view. Explain how this can happen, and how you can remedy the situation

0 Answers  

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Do you know what is recursion? Is it possible for a stored procedure to call itself or recursive stored procedure? How many levels of sp nesting is possible?

0 Answers  

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Can sql server be linked with other servers like oracle?

0 Answers  

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Can you tell me about the concept of ER diagrams?

0 Answers  

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List the various tools available for performance tuning?

0 Answers  

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What is the difference between the following two sql statements select count(*) from <tablename> select count(col_name) from <tablename>

5 Answers   247Customer,

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