Computer Science Test - 6

The correct answer is Option 4.

Key Points

The CREATE TABLE statement is used to create a new table in a database.

Create table Syntax: 

CREATE TABLE tablename(

attributename1 datatype constraint,

attributename2 datatype constraint,

:

attributenameN datatype constraint

PRIMARY KEY( one or more columns )

);

Example of a Student Table:

CREATE TABLE Student (

    StudentID int,

    LastName varchar(255),

    FirstName varchar(255),

    Class int,

    City varchar(255)

    PRIMARY KEY (StudentID)

);

The attributename parameters specify the names of the columns of the table.

The datatype parameter specifies the type of data the column can hold (e.g. integer, date, varchar, etc.).

Considering

option 1: CREATE DATABASE tablename(......) //here it violates the syntax for creating table//

option 2: CREATE ___ tablename(......) //here it misses the keyword TABLE//

option 3: CREATE TABLE tablename( datatype attributename1  constraint, ........................) //here it violates the syntax by assigning datatype first before attribute name//

The SELECT DISTINCT statement is used to return only distinct (different) values.

Syntax

SELECT DISTINCT column1, column2, ... FROM table_name;

For example

There is a table name Student_details having few attributes

Without DISTINCT

The following statement returns all cities of all Students in the Student_details tables:

SELECT city FROM Student_details;

As you can see clearly from the output, the cities are duplicate.

With DISTINCT

SELECT DISTINCT city FROM Student_details

Query Syntax for listing the tables in SQL

• Show all tables owned by the current user

SELECT  table_name FROM  user_tables;

• Show all tables in the current database

SELECT  table_name FROM  dba_tables;

• Show all tables that are accessible by the current user

SELECT  table_name FROM  all_tables;

Therefore, Correct answer is Option 1

Concept:

A transaction can be defined as a unit of part of any program at the time of its execution. During this, data items can be read or updated or both. Transaction management is the ability of database management system to manage the different transactions that occur within it.

Explanation:

Concurrency control is the activity of coordinating the actions of transactions that operate simultaneously to access the shared data. Transaction control language commands are used to manage the transaction in the database. These commands are:

COMMIT:

It is used to permanently save any transaction in the database. When changes are done in the transaction , then those changes are not permanent until we perform the COMMIT operation on that.

ROLLBACK:

This command restores the database to the last committed state. If sometimes after changing the data, we realize that changes are not required, then ROLLBACK command is applied at that point.

SAVEPOINT:

This is used to temporarily save a transaction so that rollback can be performed whenever required. Syntax for this is : SAVEPOINT savepoint_name;

The correct answer is option 4.

Concept:

The INSERT INTO statement is used to insert new records in a table and the rows can either be inserted into a table one at a time or in groups. The SQL Insert command is possible to write the INSERT INTO statement in two ways:

Specify both the column names and the values to be inserted in the group of attributes in the table.

Syntax:

INSERT INTO table_name (column1, column2, column3, ...)

VALUES (value1, value2, value3, ...);

The column names do not need to be specified in the SQL query if we are adding values to all of the table's columns. Make sure the values are in the same order as the table columns and the rows inserted into a table one at a time

Syntax:

INSERT INTO table_name

VALUES (value1, value2, value3, ...);

Hence the correct answer is rows can either be inserted into a table one at a time or in groups.

Concept:

Comments can make your application easier for you to read and maintain. For example, you can include a comment in a statement that describes the purpose of the statement within your application. With the exception of hints, comments within SQL statements do not affect the statement execution.

A comment can appear between any keywords, parameters, or punctuation marks in a statement. You can include a comment in a statement in two ways:

• Begin the comment with a slash and an asterisk (/*). Proceed with the text of the comment. This text can span multiple lines. End the comment with an asterisk and a slash (*/). The opening and terminating characters need not be separated from the text by a space or a line break.

• Begin the comment with -- (two hyphens). Proceed with the text of the comment. This text cannot extend to a new line. End the comment with a line break.

Concept:

The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.

The ALTER TABLE statement is also used to add and drop various constraints on an existing table.

ADD Column

To add a column in a table, use the following syntax:

ALTER TABLE table_name ADD column_name datatype;

DROP COLUMN

To delete a column in a table, use the following syntax (notice that some database systems don't allow deleting a column):

ALTER TABLE table_name DROP COLUMN column_name;

Concept:

The GROUP BY statement groups rows that have the same values into summary rows, like "find the number of customers in each country".

The GROUP BY statement is often used with aggregate functions (COUNT, MAX, MIN, SUM, AVG) to group the result-set by one or more columns.

GROUP BY Syntax

SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
ORDER BY column_name(s);

The ORDER BY keyword is used to sort the result-set in ascending or descending order.

The ORDER BY keyword sorts the records in ascending order by default. To sort the records in descending order, use the DESC keyword.

ORDER BY Syntax

SELECT column1, column2, ...
FROM table_name
ORDER BY column1, column2, ... ASC|DESC;

Concept:

The GROUP BY statement groups rows that have the same values into summary rows, like "find the number of customers in each country".

The GROUP BY statement is often used with aggregate functions (COUNT, MAX, MIN, SUM, AVG) to group the result-set by one or more columns.

GROUP BY Syntax

SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
ORDER BY column_name(s);

The ORDER BY keyword is used to sort the result-set in ascending or descending order.

The ORDER BY keyword sorts the records in ascending order by default. To sort the records in descending order, use the DESC keyword.

ORDER BY Syntax

SELECT column1, column2, ...
FROM table_name
ORDER BY column1, column2, ... ASC|DESC;

The HAVING clause was added to SQL because the WHERE keyword could not be used with aggregate functions.

HAVING Syntax

SELECT column_name(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
HAVING condition
ORDER BY column_name(s);

The SELECT statement is used to select data from a database.

The data returned is stored in a result table, called the result-set. 

SELECT Syntax

SELECT column1, column2, ...
FROM table_name;

Hence Option C is correct

• In SQL, the function - avg, min, max, sum, count is called as aggregate function.
• Aggregate functions return a single value after calculating from a group of values.
• avg() function returns the average values, max() returns the maximum value, count() finds the number of rows that matches the specified condition, min() returns the minimum value, Max() returns the maximum value. sum() returns the sum of values.

Example:

SELECT AVG(salary

FROM employees

GROUP BY department_id;