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Name ResolutionIn potentially uncertain SQL statements, the names of the database columns take precedence over the names of the local variables and formal parameters. For e.g. the DELETE statement removes all the employees from the emp table, not just ’KING’, as Oracle suppose that both enames in the WHERE clause refer to thedatabase column:DECLAREename VARCHAR2(10) := ’KING’;BEGINDELETE FROM emp WHERE ename = ename;In such cases, to avoid the uncertainty, prefix the names of the local variables and formal parameters with my_, as shown below:DECLAREmy_ename VARCHAR2(10);Or, use a block label to qualify references, as in<>DECLAREename VARCHAR2(10) := ’KING’;BEGINDELETE FROM emp WHERE ename = main.ename;The next illustration shows that you can use a subprogram name to qualify references to a local variables and formal parameters:FUNCTION bonus (deptno IN NUMBER, ...) RETURN REAL ISjob CHAR(10);BEGINSELECT ... WHERE deptno = bonus.deptno AND job = bonus.job;
Cause of Indeterminacy in SQL One root cause of indeterminacy in SQL lies in its implementation of comparison for equality. For certain system-defined types it is possible for
%NOTFOUND The %NOTFOUND is logical, opposite of the %FOUND. The %NOTFOUND yields FALSE if the last fetch returned a row, or TRUE when the final fetch failed to return a row. I
Relational Algebra - SQL It describes some operators, that together constitute an algebra that is not only relationally complete but also irreducibly so (very nearly- apart f
Controlling Cursor Variables You use 3 statements to control the cursor variable: OPEN-FOR, FETCH, & CLOSE. At First, you OPEN a cursor variable FOR a multi-row query. Then, y
Example of Using Aggregation on Nested Tables Example: How many students sat each exam WITH C_ER AS (SELECT CourseId, CAST (TABLE (SELECT DISTINCT StudentId, Mark FROM EXAM
Closing a Cursor The CLOSE statements disable the cursor, and the result set becomes undefined. An illustration of the CLOSE statement as shown: CLOSE c1;
Effects of NULL for Multiple Assignments - SQL If the row expression given as the source for a multiple assignment evaluates to NULL, then NULL is assigned to each target. If
UNION without CORRESPONDING - SQL The use of UNION without CORRESPONDING. Example is merely by omitting CORRESPONDING, but only because the operands have identical SELECT clau
Find the account numbers of all customers whose balance is more than 10,000 $
Multiset types - SQL An SQL multiset is what in mathematics is also known as a bag-something like a set except that the same element can appear more than once. The body of an
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