2.4 Expressions

Although all DB Access Modules are implemented through the same interface, there are still differences in the parameters that must be passed to certain expressions. The following section outlines the differences in the DB Access Module for MySQL.

Table 4 lists restrictions on the global functions associated with RWDBExpr.

rwdbAvg(const RWDBExpr&)

rwdbCast(const RWDBExpr&,
         const RWDBValue&)

rwdbCast(const RWDBExpr&,
         const RWDBValue&,
         const RWDBExpr&)

rwdbCast(const RWDBExpr&,
         const RWDBValue&,
         const RWDBExpr&, 
         const RWDBExpr&)

rwdbCharLength(const RWDBExpr&)

rwdbCount()

rwdbCount(const RWDBExpr&)

rwdbCountDistinct(const RWDBExpr&)

rwdbCurrentUser()

rwdbExists(const RWDBSelectorBase&)

rwdbLower(const RWDBExpr&)

rwdbMax(const RWDBExpr&)

rwdbMin(const RWDBExpr&)

rwdbName(const RWCString&,
         const RWDBExpr&)

rwdbPosition(const RWDBExpr&,
             const RWDBExpr&)

rwdbSessionUser()

rwdbSubString(const RWDBExpr&,
              const RWDBExpr&)

rwdbSubString(const RWDBExpr&,
              const RWDBExpr&,
              const RWDBExpr&)

rwdbSum(const RWDBExpr&)

rwdbSystemDateTime()

rwdbSystemUser()

rwdbTrimLeading(const RWDBExpr&,
                const RWDBExpr&)

rwdbTrimTrailing(const RWDBExpr&,
                 const RWDBExpr&)

rwdbTrimBoth(const RWDBExpr&,
             const RWDBExpr&)

rwdbUpper(const RWDBExpr&)

For the DB Access Module for MySQL, the DB Interface Module supports the writing of outer joins using ANSI SQL 92 syntax only. See the examples on constructing outer joins in Section 5.3.3.1, "Outer Join Constructs In the FROM Clause (ANSI-Compliant)," in the DB Interface Module User's Guide.

The following example shows how you would write an outer join program for MySQL. You can assume that myDbase is a valid instance of RWDBDatabase.

Example 2: An Outer Join for MySQL Using ANSI-Compliant Syntax

RWDBTable employee = myDbase.table("emp"); RWDBTable depart = myDbase.table("dept"); RWDBTable locate = myDbase.table("loc"); RWDBSelector selector = myDbase.selector(); selector << employee["empnum"] << employee["ename"] << employee["deptno"] << depart["deptno"] << depart["dname"] << depart["locno"] << locate["locno"] << locate["lname"]; // Define and declare join1 as a right outer join // between the tables emp and dept. RWDBJoinExpr join1 = rwdbRightOuter(employee, depart); // Attach the join criterion to join1 using // the on() method. This criterion joins the two tables // emp and dept by their deptno columns. join1.on(employee["deptno"] == depart["deptno"]); // Define and declare join2 as a left outer join // between the table loc and join1, forming a nested join. // Note that the nested join must be specified as second // parameter and not first. RWDBJoinExpr join2 = rwdbLeftOuter(locate, join1); // Attach the join criterion to the join2 using // the on() method. This criterion joins the two tables // dept and loc by their locno columns. join2.on(depart["locno"] == locate["locno"]); // Attach join2 explicitly to the selector FROM clause // using the from() method. selector.from(join2);

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