Assigning New Values to Table Columns

SourcePro : DB Interface Module User’s Guide : PART II Using the Basic Features : The Data Manipulation Classes : Expressions, Criteria, and Assignments : Assigning New Values to Table Columns

Use RWDBAssignment to assign new values to columns in tables, in other words, to update. The only way to create an instance of class RWDBAssignment is through the invocation of the RWDBColumn::assign() method. An RWDBAssignment is an encapsulation of the SQL phrase:

SET column = expression

where column refers to the RWDBColumn instance whose assign() method produced the RWDBAssignment, and expression refers to its argument. Here is an example:

Example 6 – Assigning a value to a column

RWDBAssignment a = col1.assign(col2 / 3.1415);

This is equivalent to the SQL phrase:

SET COL1 = COL2/3.1415

The compiler interprets the SQL phrase as follows:

The assign() member function accepts an RWDBExpr as an argument.

The compiler does not find an overloaded operator / for RWDBColumn instances and doubles, but it has one for two RWDBExpr instances.

Since the compiler can create an RWDBExpr instance from both an RWDBColumn and a double, it does.

It then applies the operator / to form an RWDBExpr acceptable to the assign() function.

The instance of RWDBAssignment can then be used with the RWDBUpdater class. See Updating Data.

Eventually, the assignment is executed by the underlying access library module.