Database and SQL

Create is a DDL statement as this statement defines the structure of a database.

The REVOKE command revokes previously granted privileges from one or more roles.

SQL GRANT is a command used to provide access or privileges of the database objects to other users.

This option is correct as CREATE TRIGGER command is a SQL statement that creates a trigger to the database schema.

This option is correct as AVG is a predefined SQL function that is used to calculate the average of a given record.

This option is correct as transaction control commands manage the changes made by data manipulation commands.

This option is correct as this distinct keyword selects only those values that are unique.

This option is correct as primary key constraint does not allow duplicacy of data. Hence, it uniquely identifies each row in the table.

The alter statement is used to resize a column.  The size of a column can both be increased or decreased using the alter keyword. Normally, the size of a column can be increased to a large value but it can only be decreased to the limit that if there is data present in the column, it should not get trimmed.

It is the language to handle basic database operations like creating a table and managing it. Some of the keywords of SQL are Create,Select, Delete, Update etc.. SQL is also sometimes called SEQUEL which stands for Simple English Like Query Language. SQL is supported by all the major RDBMS products like Oracle, Ms-SQL Server, MySQL to name a few.

The Like operator is used for pattern matching in VARCHAR columns. It is useful in conditions where we do not to compare a value exactly with another. For example, if we want to search for all the persons whose names start with  A, we can use the Like operator with the wildcard character. % is used for substituting a string. For conducting a character replacement we can use the _ (underscore) operator. It is used for matching values that are of a particular length.

The above statement is a valid statement and will display a list of tables available in a user's account. The statement will also print the names of the available view, synonyms etc. The select * from tab statement actually queries a synonym named tab which is in the System user account. All the users created in Oracle have a default access to the synonym. Therefore when we are issuing the above statement we are in fact querying the said synonym.

Rollback is a part of Transaction Control Language or (TCL). It is used to cancel or undo any changes made in a table. For example, it the user has accidently deleted the data, it can be easily rollbacked to the previous state. The other commands of the same category (TCL) are commit, savepoint etc.

Alias is the temporary name given to a table or a column. An alias is quite useful where the name of the table is lengthy and the name has to be written more than once in a statement. A table alias is generally a word with one or two letters that can substitute a table name. For example, if the name of a table is sales_1999, then instead of writing sales_1999 everytime it is required, we can create an alias an use it. Consider the following statement: Select s.data from sales_1999 where s.no=101 in the above statement. S is an alias and it is used as a substitution for the word sales_1999.

The Truncate Table statement is used to delete all the rows of a table at once. The statement allows the user to delete all the records without specifying a condition using the where clause. Truncate  deletes all the records but keeps the structure of the table intact. The output of this statement is equivalent to the delete statement without the where statement.