SQL Schema Evolution and DROP Command
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SQL Schema Evolution and DROP Command

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Questions and Answers

What is the purpose of schema evolution commands in SQL?

  • To change the schema while the database is operational (correct)
  • To drop the entire database
  • To create a new database schema
  • To recompile the database schema
  • What is the main function of the DROP command in SQL?

  • To create a new constraint
  • To alter a column definition
  • To drop named schema elements, such as tables or domains (correct)
  • To add a new column to a table
  • What is the effect of using the CASCADE option with the DROP command?

  • It does nothing
  • It creates a new schema element
  • It removes only the specified schema element
  • It removes the schema element and all its dependent elements (correct)
  • What is the purpose of the ALTER table command in SQL?

    <p>All of the above</p> Signup and view all the answers

    What is the effect of adding a constraint to a table using the ALTER table command?

    <p>It adds a constraint to the table</p> Signup and view all the answers

    How can you drop a column from a table using the ALTER table command?

    <p>Using the ALTER command with the CASCADE option</p> Signup and view all the answers

    What happens when you drop a default value using the ALTER table command?

    <p>The default value is dropped and the column is set to null</p> Signup and view all the answers

    What is the purpose of the RESTRICT option with the DROP command?

    <p>To drop the column only if it is not referenced in any views</p> Signup and view all the answers

    What is the effect of using the CASCADE option with the ALTER table command to drop a column?

    <p>It drops the column and all its dependent elements, such as views</p> Signup and view all the answers

    What is the purpose of the ALTER table command with the SET DEFAULT option?

    <p>To set a new default value for a column</p> Signup and view all the answers

    Study Notes

    Nested Queries

    • A nested query is a complete select-from-where block within the WHERE clause of another query
    • An outer query and nested subqueries are used to compare a value with a set (or multiset) of values
    • The IN operator is used to compare a value with a set (or multiset) of values

    Tuple Comparisons

    • Tuples of values can be used in comparisons by placing them within parentheses
    • The = ANY (or = SOME) operator returns TRUE if the value is equal to some value in the set V and is equivalent to IN
    • Other comparison operators that can be combined with ANY (or SOME) are >, >=, etc.

    Combining WHERE and HAVING Clauses

    • The WHERE clause applies tuple by tuple, whereas the HAVING clause applies to entire groups of tuples
    • The correct specification of the query is: GROUP BY Dno HAVING COUNT (*) > 5;

    WITH Clause

    • The WITH clause allows a user to define a table that will only be used in a particular query
    • It is used for convenience to create a temporary “View” and use that immediately in a query
    • It allows a more straightforward way of looking at a step-by-step query

    Recursive Queries

    • A recursive operation is used to retrieve all supervisees of a supervisory employee at all levels
    • The WITH RECURSIVE clause is used to define a recursive query
    • The query starts with an empty table and successively builds the table by computing immediate supervisees first, then second-level supervisees, and so on, until a fixed point is reached and no more supervisees can be added

    Expanded Block Structure of SQL Queries

    • The expanded block structure of SQL queries includes SELECT, FROM, WHERE, GROUP BY, and HAVING clauses

    Specifying Constraints and Actions

    • Semantic constraints are those beyond the scope of the EER and relational model
    • CREATE ASSERTION is used to specify additional types of constraints outside the scope of built-in relational model constraints
    • CREATE TRIGGER is used to specify automatic actions that the database system will perform when certain events and conditions occur

    Triggers

    • A trigger has three components: Event(s), Condition, and Action
    • Triggers are used to monitor the database and perform automatic actions when certain events and conditions occur

    Views

    • A view is a virtual table derived from other tables called the defining tables
    • A view is considered to be a virtual table that is not necessarily populated
    • CREATE VIEW command is used to specify the contents of the view

    Update on Views

    • An update on a view defined on a single table without any aggregate functions can be mapped to an update on the underlying base table
    • Updates are not permitted on aggregate views

    View Update and Inline Views

    • Views involving joins often cannot be updated because it is not possible for the DBMS to determine which of the updates is intended
    • The WITH CHECK OPTION clause must be added at the end of the view definition if a view is to be updated to make sure that tuples being updated stay in the view
    • Inline views are defined in the FROM clause of an SQL query

    Views as Authorization Mechanism

    • Views can be used to hide certain attributes or tuples from unauthorized users
    • SQL query authorization statements (GRANT and REVOKE) are used to control access to views

    Schema Change Statements

    • Schema evolution commands are used to change the schema while the database is operational
    • Schema evolution commands do not require recompilation of the database schema
    • DROP, ALTER, and CREATE commands are used to change the schema

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    Description

    Learn about schema change statements in SQL, including schema evolution commands and the DROP command used to drop named schema elements.

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