To remove duplicate rows from the result set of a query, the DISTINCT keyword is specified after SELECT.
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DISTINCT
The DISTINCT keyword ensures that each row in the result set is unique, eliminating duplicates.
Question 2
The WHERE clause is used to filter records based on a specified condition, returning only the rows that satisfy it.
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WHERE
The WHERE clause applies a filter to individual rows before they are grouped or returned.
Question 3
An aggregate function like SUM is commonly used with the GROUP BY clause to perform calculations on sets of rows.
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SUM
The SUM function calculates the total value of a numeric column for each group defined by the GROUP BY clause.
Question 4
An INNER JOIN returns only the rows that have matching values in both tables involved in the join operation.
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INNER
An INNER JOIN combines rows from two or more tables where there is a match in the specified columns from both tables.
Question 5
After grouping data, the HAVING clause is used to filter these groups based on a specified condition involving aggregate functions.
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HAVING
The HAVING clause filters groups created by the GROUP BY clause, similar to how WHERE filters individual rows.
Question 6
A subquery, also known as an inner query, is a query nested inside another SQL query statement.
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subquery
A subquery is a query embedded within another query, used to return data that will be used by the outer query.
Question 7
The EXISTS operator is often used with subqueries to check for the presence of any rows returned by the subquery, without retrieving the data itself.
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EXISTS
The EXISTS operator returns true if the subquery returns one or more rows, and false otherwise, making it efficient for checking existence.
Question 8
To improve the speed of data retrieval operations on a column, a database index can be created.
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index
An index is a special lookup table that the database search engine can use to speed up data retrieval.
Question 9
A clustered index determines the physical order of data storage in a table, meaning the table can only have one such index.
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clustered
A clustered index reorders the physical storage of the table's rows according to the index key, making data retrieval very fast for range queries.
Question 10
The UNIQUE constraint ensures that all values in a column or a set of columns are distinct across all rows.
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UNIQUE
The UNIQUE constraint prevents duplicate values in the specified column(s), maintaining data integrity.
Question 11
The EXPLAIN command is a crucial tool for query optimization, showing the execution plan chosen by the database engine.
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EXPLAIN
The EXPLAIN command (or EXPLAIN ANALYZE) allows users to see how the database optimizer plans to execute a query, helping identify performance bottlenecks.
Question 12
A cost-based optimizer analyzes various execution plans for a query and selects the one with the lowest estimated cost.
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optimizer
A cost-based optimizer uses statistics about the data to estimate the cost of different query execution plans and chooses the most efficient one.
Question 13
Database statistics provide the optimizer with information about the data distribution and cardinality, which is vital for efficient query plan selection.
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statistics
Database statistics are crucial for the query optimizer to make informed decisions about the most efficient way to retrieve data.