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Effects of NULL
The numeric variable X, perhaps of type INTEGER, might be assigned NULL. In that case the result of evaluating X + 1 is NULL, and so SET Y = X + 1 assigns NULL to Y. A curious consequence of this is that immediately following this assignment the comparison Y = X + 1 does not evaluate to TRUE. For that matter, neither does the comparison Y = Y. In fact they both evaluate to UNKNOWN.
Tables within a Table - SQL Figure here is an exact copy of the one in the theory book and as before it is just an alternative way of representing some of the information con
Example of WHEN or THEN Constraints A concrete example showing how SQL supports WHEN/THEN constraints CREATE TABLE SAL_HISTORY (EmpNo CHAR (6), Salary INTEGER NOT NULL,
Datatypes Every constant and variable has a datatype that specifies the storage format, constraints, and the valid range of values. The PL/SQL gives a variety of predefined dat
Use Native Dynamic SQL A few programs (a normal-purpose report writer for illustration) should build and process a variety of SQL statements at run time. Therefore, their full
Declaring Records Whenever you define a RECORD type, you may declare records of that type, as the illustration shows: DECLARE TYPE StockItem IS RECORD ( item_no INTEG
Inner Join We have learned how to retrieve data from one table by using SELECT statement. But, as we have learned, normalized relational databases mean the data is spread betw
Parameter Default Values As the illustration below shows, you can initialize the IN parameters to the default values. In that way, you can pass various numbers of actual par
Data Abstraction The Data abstraction extracts the important properties of data while ignoring the not necessary details. Once you design a data structure, you can fail to reme
Pl/SQL Expressions The Expressions are constructed by using the operands and operators. An operand is a constant, literal, variable, or function call which contributes a value
Use of COUNT in SQL It describes and discusses various general methods of expressing constraints, eventually noting that support for "=" with relation operands is sufficient f
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