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DBMS 面试题与答案

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问题 26. Whats the difference between UNION and UNION ALL?

UNION SQL syntax is used to select information from two tables. But it selects only distinct
records from both the table, while UNION ALL selects all records from both the tables.

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问题 27. What are cursors and what are the situations you will use them?

SQL statements are good for set at a time operation. So it is good at handling set of data. But
there are scenarios where we want to update row depending on certain criteria. we will loop
through all rows and update data accordingly. Theres where cursors come in to picture.

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问题 28. What is " Group by " clause?

Group by clause group similar data so that aggregate values can be derived.

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问题 29. What is the difference between HAVING and WHERE clause?

HAVING clause is used to specify filtering criteria for GROUP BY, while WHERE clause applies on normal SQL.

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问题 30. What is a Sub-Query?

A query nested inside a SELECT statement is known as a subquery and is an alternative to
complex join statements. A subquery combines data from multiple tables and returns results
that are inserted into the WHERE condition of the main query. A subquery is always enclosed within parentheses and returns a column. A subquery can also be referred to as an inner query and the main query as an outer query. JOIN gives better performance than a subquery when you have to check for the existence of records. 
For example, to retrieve all EmployeeID and CustomerID records from the ORDERS table that have the EmployeeID greater than the average of the EmployeeID field, you can create a nested query, as shown:
SELECT DISTINCT EmployeeID, CustomerID FROM ORDERS WHERE EmployeeID > (SELECT AVG(EmployeeID) FROM ORDERS)

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