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Examine this SQL statement:
DELETE FROM employees e
WHERE EXISTS
(SELECT'dummy'
FROM emp_history
WHERE employee_id = e.employee_id)
Which two are true?
The provided DELETE statement uses a correlated subquery to determine which rows should be deleted from the EMPLOYEES table.
A . The subquery is indeed executed for every row in the EMPLOYEES table. This is because it references e.employee_id, which is a column from the outer query, making it a correlated subquery.
B . The subquery is a correlated subquery, as explained above.
C . The subquery is executed for each row, not before the DELETE statement.
D . Not all existing rows in the EMPLOYEES table are deleted, only those that have a corresponding employee_id in the EMP_HISTORY table.
E . The DELETE statement executes successfully even if the subquery selects multiple rows because the EXISTS condition only checks for the presence of rows, not their count.
Oracle Database SQL Language Reference 12c Release 1 (12.1), DELETE Statement
Oracle Database SQL Language Reference 12c Release 1 (12.1), Subquery Factoring
Oracle Database SQL Language Reference 12c Release 1 (12.1), Correlated Subqueries
Which three are true about scalar subquery expressions?
Scalar subquery expressions are used in Oracle SQL to return a single value from a subquery.
Option C: They can be nested.
Scalar subqueries can indeed be nested within another scalar subquery, provided each subquery returns a single value.
Option D: A scalar subquery expression that returns zero rows evaluates to null.
According to Oracle documentation, if a scalar subquery returns no rows, the result is a NULL value, not zero or any other default.
Option F: They can be used as default values for columns in a create table statement.
Scalar subqueries can be specified in the DEFAULT clause of a column in a CREATE TABLE statement to dynamically assign default values based on the result of the subquery.
Options A, B, and E are incorrect based on Oracle SQL standards and functionalities.
The CUSTOMERS table has a CUST_CREDT_LIMIT column of data type number.
Which two queries execute successfully?
A . True - The TO_CHAR function is used correctly here to convert the numeric value to a string, and NVL handles the case where cust_credit_limit might be NULL. The expression inside NVL computes 15% of the credit limit or displays 'Not Available' if the credit limit is NULL. The syntax is correct.
B . False - The NVL2 function requires three parameters: the expression to check for NULL, the value to return if it's not NULL, and the value to return if it is NULL. The given usage lacks the required parameters and syntax.
C . False - The NVL function expects both parameters to be of the same data type. Since the second parameter 'Not Available' is a string, it causes a data type conflict with the numerical result of the first parameter.
D . False - The keyword SELECT is misspelled as SLECT, making the syntax incorrect.
Which four statements are true about constraints on Oracle tables?
C: True. A UNIQUE constraint in Oracle SQL allows for the inclusion of NULL values; specifically, it permits multiple NULLs in a column or set of columns but ensures that all non-NULL values are unique.
D: True. A PRIMARY KEY constraint can indeed be added to a table after it has been created and even after it has been populated, as long as the existing data does not violate the primary key constraint rules (i.e., all values must be unique and not NULL).
F: True. A UNIQUE constraint can utilize a pre-existing index on the columns it covers. If a suitable index already exists, Oracle can use this index to enforce the constraint, optimizing performance and resource utilization.
G: True. Columns that are part of a FOREIGN KEY constraint can contain NULL values. This is permissible under SQL standards and Oracle implementation, as a NULL foreign key value is considered to not refer to any row in the referenced table and thus does not violate referential integrity.
In your session NLS_ DATE_ FORMAT is set to DD--MON_RR.
Which two queries display the year as four digits?
A . True. The TO_CHAR function will convert SYSDATE to a string using the format 'MM/DD/YYYY', which displays the year as four digits. Then TO_DATE will convert this string back to a date without changing its appearance because the format matches.
E . True. The TO_CHAR function with the format 'MM/DD/YYYY' will display the date with the year as four digits, regardless of the NLS_DATE_FORMAT setting.
B, C, D, and F are incorrect because they either do not specify the format model needed to display the year as four digits or use TO_DATE which does not format the output but rather interprets the input.
