Already have an account? Get multiple benefits of using own account!
Login in your account..!
Remember me
Don't have an account? Create your account in less than a minutes,
Forgot password? how can I recover my password now!
Enter right registered email to receive password!
Existential Quantification - SQL
Existential quantification-stating that something is true of at least one object under consideration-can be expressed by OR(r,c), meaning that at least one object that satisfies a predicate for r also satisfies c, and IS_NOT_EMPTY(r). The names for the aggregate operators AND and OR reflect the facts that when we confine our attention to finite sets, universal and existential quantification are equivalent to repeated invocations of dyadic AND and dyadic OR, respectively. Note that AND(r,c) is equivalent to COUNT(r) = COUNT(r WHERE c), and OR(r,c) is equivalent to COUNT(r WHERE c) > 0 and also to IS_NOT_EMPTY(r WHERE c).
Quantification also appears in various guises in SQL, but its meaning is muddied by those same two violations of relational theory that we have already seen muddying the waters: duplicate rows and NULL. For example, SQL's (SELECT COUNT(*) FROM r), a so-called scalar subquery (because it is an expression denoting a table with one row and one column, enclosed in parentheses), denotes the number of rows in the table r, but can we really say that this represents the number of objects that satisfy a predicate for r, if the same row can be counted more than once, or if NULL appears in place of a column value in some row of r? In fact, what might it mean to say that a row does or does not satisfy a predicate? In 2VL we say that object a satisfies predicate P(x) exactly when P(a) is true. Does this still hold in 3VL, or might SQL deem a to satisfy P(x) also when P(a) is unknown? Well, it turns out that SQL uses both interpretations, depending on the context, as we shall discover.
Effect of Anonymous Columns Now, recall that a VALUES expression denotes a table with undefined column names. If an initial value is to be specified when a base table is creat
Tautologies: Above given table allows us to read the truth of the connectives in the next manner. Just expect we are looking at row three. It means this says that, if there P
a. Write an anonymous block that contains a PL/SQL function. Given an order number orderNo, the function will calculate the total number of the parts in the order. Then the anonym
Controlling Autonomous Transactions The first SQL statement in an autonomous routine starts a transaction. Whenever one transaction ends, the next SQL statement starts the oth
Some Varray Examples In SQL Plus, assume that you define an object type Project, as described below: SQL> CREATE TYPE Project AS OBJECT ( 2 project_no NUMBER(2), 3 title VARCHA
Explicitly specifying the join condition - SQL SELECT * FROM IS_CALLED JOIN IS_ENROLLED_ON ON ( IS_CALLED.StudentId = IS_ENROLLED_ON.StudentId ) Now, the key word JO
Raise_application_error - procedure of package DBMS_STANDARD , allows to issue an user_defined error messages by stored sub-program or database trigger.
Natural Join - SQL In the absence of NATURAL JOIN Example has to be replaced by something rather more longwinded, as shown in Example. Example: Joining IS_CALLED and IS_EN
Parameter and Keyword Description: cursor_variable_name: This identifies a cursor variable or the parameter formerly declared within the present scope. host_cursor_va
Example of COALESCE operator Example: Give the total of marks for each exam (simplified solution) SELECT CourseId, COALESCE ((SELECT SUM (Mark) FROM EXAM_MARK AS EM
Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!
whatsapp: +91-977-207-8620
Phone: +91-977-207-8620
Email: [email protected]
All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd