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Questions and Answers
What is another name for a nested query in SQL?
What is another name for a nested query in SQL?
The ALL operator in SQL returns TRUE if at least one of the subquery values meets the condition.
The ALL operator in SQL returns TRUE if at least one of the subquery values meets the condition.
False
In which clause are subqueries most commonly added?
In which clause are subqueries most commonly added?
WHERE clause
A __________ is written inside another SQL query.
A __________ is written inside another SQL query.
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Match the following SQL operators with their functions:
Match the following SQL operators with their functions:
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Which statement is true regarding the ANY operator?
Which statement is true regarding the ANY operator?
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In MySQL, a subquery can be nested inside a SELECT statement.
In MySQL, a subquery can be nested inside a SELECT statement.
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What must a subquery be enclosed in?
What must a subquery be enclosed in?
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A subquery can occur within a SELECT, __________, or WHERE clause.
A subquery can occur within a SELECT, __________, or WHERE clause.
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Which of the following uses the ALL operator in SQL?
Which of the following uses the ALL operator in SQL?
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What does the SQL keyword ANY do in a query?
What does the SQL keyword ANY do in a query?
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In the context of SQL, what is the key difference between ANY and ALL?
In the context of SQL, what is the key difference between ANY and ALL?
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The WHERE clause applies to entire groups of tuples while HAVING applies tuple by tuple.
The WHERE clause applies to entire groups of tuples while HAVING applies tuple by tuple.
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Which SQL clause should you use when you want to filter results after grouping?
Which SQL clause should you use when you want to filter results after grouping?
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A salary of ______ satisfies the condition salary > ALL(3000, 4000, 5000).
A salary of ______ satisfies the condition salary > ALL(3000, 4000, 5000).
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Match the SQL keywords with their descriptions:
Match the SQL keywords with their descriptions:
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What type of values does the ANY condition compare against?
What type of values does the ANY condition compare against?
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Which of the following SQL statements uses the ANY keyword correctly?
Which of the following SQL statements uses the ANY keyword correctly?
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The GROUP BY clause is used to create groups of tuples in SQL.
The GROUP BY clause is used to create groups of tuples in SQL.
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To find employees who earn more than all employees in the Sales department, the query starts with: SELECT name, salary FROM employees WHERE salary > ______.
To find employees who earn more than all employees in the Sales department, the query starts with: SELECT name, salary FROM employees WHERE salary > ______.
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Study Notes
SQL-2 and Normalization
- SQL-2 includes joins, nested queries (correlated and uncorrelated), and aggregate functions (group by and having clauses).
- Normalization focuses on functional dependencies, good relational database design, and atomic domains.
- Nested queries are queries written inside another query, also called subqueries or inner queries.
- A subquery's result is used in the outer query's execution.
- Subqueries can be placed in the SELECT, FROM, or WHERE clause.
- MySQL subqueries can be nested inside SELECT, INSERT, UPDATE, DELETE, or SET statements.
- Nested queries can be independent or correlated.
- Independent queries run independently.
- Correlated queries depend on the outer query for execution.
- Various comparison operators (e.g., =, >, <, IN, NOT IN, ANY, ALL) are used in subqueries.
- Subqueries can return scalar, row, column, or table values.
- Scalar subqueries return a single value.
- A subquery returning 0 rows returns NULL.
- Multiple rows returned by a subquery with an operator other than "=", "=", "IN" or "NOT IN" will cause an error.
- SQL uses three-valued logic (TRUE, FALSE, and UNKNOWN).
- NULL values may cause unexpected results during comparisons
- The EXISTS operator in a correlated subquery determines if a subquery returns rows, and NOT EXISTS checks whether the subquery doesn't return rows.
- The SQL function UNIQUE evaluates if there are any duplicate values in the result of a query.
Nested Queries
- A subquery can return a scalar (single value), a single row, a single column, or a table.
- Comparisons with subqueries use operators like < , >, =, NOT EQUAL TO, etc.
- Subqueries are placed within parentheses, in the SELECT, WHERE, or HAVING clause of a query.
- Comparison operators involving NULL values return UNKNOWN.
Types of Nested Queries
- Independent Nested Queries: The execution of the inner query is independent of the outer query.
- Correlated Nested Queries: The execution of the inner query depends on the outer query.
- Using the IN operator in a subquery returns TRUE if the value is one of the values within the set.
Subqueries with IN, ALL, ANY, or SOME
- Subquery after a comparison operator (e.g., =, >, < ,etc.,) followed by ALL, ANY, or SOME are used with multiple-row operators.
- IN operator in a nested query is similar to ANY or SOME.
- In general, ALL comparisons can be combined with each operator.
- The keyword SOME has the same meaning as ANY.
- The ALL comparison condition (e.g., v > ALL V) returns TRUE if v is greater than all the values in the set V.
- The ANY comparison condition (e.g., v > ANY V) returns TRUE if v is greater than any value in the set V.
Subqueries with EXISTS or NOT EXISTS
- EXISTS and NOT EXISTS are used to check for the presence or absence of rows in a subquery.
- If a subquery inside EXISTS returns any rows, the condition in the outer query evaluates to TRUE.
- If the subquery inside NOT EXISTS does not return any rows, the condition in the outer query evaluates to TRUE.
JOIN vs SUBQUERY
- For distributed databases, nested queries are better, but joins use more space.
- Joins are faster, and they are universally understood, thus no optimization issues arise, but they consume more space.
Different Types of JOINed Tables
- NATURAL JOIN on two relations (e.g., R and S): No join condition is specified. It's equivalent to an implicit EQUIJOIN condition for each pair of attributes with the same name.
INNER and OUTER JOINs
- INNER JOIN: The default type; a tuple is included in the result only if a matching tuple exists in the other relation.
- LEFT OUTER JOIN: Every tuple in the left table must also appear in the result even without a matching tuple from the right table; it is padded with NULL.
- RIGHT OUTER JOIN: Every tuple in the right table must appear in the result even without a matching tuple from the left table and is padded with NULL.
Multiway JOIN in the FROM clause
- Can nest JOIN specifications in a multiway join.
Views in SQL
- A view in SQL is a virtual table derived from other tables (base tables or other views).
- Views are not stored physically, but are derived from a select statement.
- Views are a virtual relation based on the result set of a SELECT statement.
More Complex SQL Retrieval Queries
- Additional features allow for complex retrieval from the database: nested queries, joined tables, outer joins (in the FROM clause), aggregate functions, and grouping.
Comparisons Involving NULL and Three-Valued Logic
- A NULL value is considered unknown, and different from other NULL values.
- Comparisons involving NULL values return UNKNOWN.
- SQL uses three-valued logic: TRUE, FALSE, and UNKNOWN.
Additional SQL Concepts
- Aggregate functions (COUNT, SUM, MAX, MIN, and AVG) will exclude NULL values.
- Aggregate functions are used for summarizing data from multiple tuples into a single tuple (e.g., to find a total or average).
- Grouping clauses such as GROUP BY are used in aggregate functions to group data before calculation.
- The HAVING clause specifies a condition to select or reject a group of tuples.
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Description
This quiz covers essential topics in SQL-2, including joins, nested queries, and aggregate functions. It also addresses normalization, focusing on functional dependencies and relational database design principles. Test your understanding of subqueries and their applications in SQL.
