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.
Global Functions
The DB Interface Module provides a predefined set of global functions, which allow an application to build portable SQL expressions.
Table 4 lists restrictions on the global functions associated with
RWDBExpr.
Table 4 – Restrictions on the use of global functions with RWDBExpr
Function | Restrictions |
rwdbAvg(const RWDBExpr&) | None. Corresponds to: AVG(expr) |
rwdbCast(const RWDBExpr&,
const RWDBValue&) | None. Corresponds to: {fn CONVERT(expr, value)} |
rwdbCast(const RWDBExpr&,
const RWDBValue&,
const RWDBExpr&) | Same as the two-expression form, as MySQL does not support this form. |
rwdbCast(const RWDBExpr&,
const RWDBValue&,
const RWDBExpr&,
const RWDBExpr&) | Same as the two-expression form, as MySQL does not support this form. |
rwdbCharLength(const RWDBExpr&) | None. Corresponds to: {fn LENGTH(expr)} |
rwdbCount() | None. Corresponds to: COUNT(*) |
rwdbCount(const RWDBExpr&) | None. Corresponds to: COUNT(expr) |
rwdbCountDistinct(const RWDBExpr&) | None. Corresponds to: COUNT(DISTINCT expr) |
rwdbCurrentUser() | None. Corresponds to: CURRENT_USER() |
rwdbExists(const RWDBSelectorBase&) | None. Corresponds to: EXISTS(sel) |
rwdbLower(const RWDBExpr&) | None. Corresponds to: {fn LCASE(expr)} |
rwdbMax(const RWDBExpr&) | None. Corresponds to: MAX(expr) |
rwdbMin(const RWDBExpr&) | None. Corresponds to: MIN(expr) |
rwdbName(const RWCString&,
const RWDBExpr&) | None. Corresponds to: expr AS str |
rwdbPosition(const RWDBExpr&,
const RWDBExpr&) | None. Corresponds to: {fn LOCATE(expr0, expr1)} |
rwdbSessionUser() | None. Corresponds to: SESSION_USER() |
rwdbSubString(const RWDBExpr&,
const RWDBExpr&) | None. Corresponds to: {fn SUBSTRING(expr0, expr1, LENGTH(expr0))} |
rwdbSubString(const RWDBExpr&,
const RWDBExpr&,
const RWDBExpr&) | None. Corresponds to: {fn SUBSTRING(expr0, expr1, expr2)} |
rwdbSum(const RWDBExpr&) | None. Corresponds to: SUM(expr) |
rwdbSystemDateTime() | None. Corresponds to: {fn NOW()} |
rwdbSystemUser() | None. Corresponds to: SYSTEM_USER() |
rwdbTrimLeading(const RWDBExpr&,
const RWDBExpr&) | None. Corresponds to: TRIM(LEADING expr0 FROM expr1) |
rwdbTrimTrailing(const RWDBExpr&,
const RWDBExpr&) | None. Corresponds to: TRIM(TRAILING expr0 FROM expr1) |
rwdbTrimBoth(const RWDBExpr&,
const RWDBExpr&) | None. Corresponds to: TRIM(BOTH expr0 FROM expr1) |
rwdbUpper(const RWDBExpr&) | None. Corresponds to: {fn UCASE(expr)} |
Outer Joins
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 the section
“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);