// Module included in the following assemblies:
// as_procedure-language.adoc
[id="dynamic-sql-command"]
= Dynamic SQL command

Dynamic SQL allows for the execution of an arbitrary SQL command in a virtual procedure. 
Dynamic SQL is useful in situations where the exact command form is not known prior to execution.

.Usage

[source,sql]
----
EXECUTE IMMEDIATE <sql expression> AS <variable> <type> [, <variable> <type>]* [INTO <variable>] [USING <variable>=<expression> [,<variable>=<expression>]*] [UPDATE <literal>]
----

.Syntax rules

* The SQL expression must be a CLOB or string value of less than 262144 characters.
* The `AS` clause is used to define the projected symbols names and types returned by the executed SQL string. 
The `AS` clause symbols will be matched positionally with the symbols returned by the executed SQL string. 
Non-convertible types or too few columns returned by the executed SQL string will result in an error.
* The `INTO` clause will project the dynamic SQL into the specified temp table. 
With the `INTO` clause specified, the dynamic command will actually execute a statement that behaves like an INSERT with a QUERY EXPRESSION. 
If the dynamic SQL command creates a temporary table with the `INTO` clause, then the `AS` clause is required to define the table’s metadata.  Note that if the temporary table already exists, then the insert columns are matched positionally - not by name.
* The `USING` clause allows the dynamic SQL string to contain variable references that are bound at runtime to specified values. 
This allows for some independence of the SQL string from the surrounding procedure variable names and input names. 
In the dynamic command `USING` clause, each variable is specified by short name only. 
However, in the dynamic SQL the `USING` variable must be fully qualified to `DVAR`. 
The `USING` clause is only for values that will be used in the dynamic SQL as valid expressions. 
It is not possible to use the `USING` clause to replace table names, keywords, and so forth. 
This makes using symbols equivalent in power to normal bind (?) expressions in prepared statements. 
The `USING` clause helps reduce the amount of string manipulation needed. 
If a reference is made to a USING symbol in the SQL string that is not bound to a value in the `USING` clause, an exception will occur.
* The `UPDATE` clause is used to specify the updating model count.  
Accepted values are (0,1,*). 
0 is the default value if the clause is not specified.
For more information, see {% if book.targetDVProd %}xref:updating-model-count{% else %}link:r_updating-model-count.adoc{% endif %}[Updating model count].

[source,sql]
.*Example: Dynamic SQL*
----
...
/* Typically complex criteria would be formed based upon inputs to the procedure.
 In this simple example the criteria is references the using clause to isolate
 the SQL string from referencing a value from the procedure directly */

DECLARE string criteria = 'Customer.Accounts.Last = DVARS.LastName';

/* Now we create the desired SQL string */
DECLARE string sql_string = 'SELECT ID, First || " " || Last AS Name, Birthdate FROM Customer.Accounts WHERE ' || criteria;

/* The execution of the SQL string will create the #temp table with the columns (ID, Name, Birthdate).
  Note that we also have the USING clause to bind a value to LastName, which is referenced in the criteria. */
EXECUTE IMMEDIATE sql_string AS ID integer, Name string, Birthdate date INTO #temp USING LastName='some name';

/* The temp table can now be used with the values from the Dynamic SQL */
loop on (SELCT ID from #temp) as myCursor
...
----

Here is an example showing a more complex approach to building criteria for the dynamic SQL string. 
In short, the virtual procedure `AccountAccess.GetAccounts` has the inputs `ID`, `LastName`, and `bday`. 
If a value is specified for `ID` it will be the only value used in the dynamic SQL criteria. 
Otherwise, if a value is specified for `LastName` the procedure will detect if the value is a search string. 
If `bday` is specified in addition to `LastName`, it will be used to form compound criteria with `LastName`.

[source,sql]
.*Example: Dynamic SQL with USING clause and dynamically built criteria string*
----
...
DECLARE string crit = null;

IF (AccountAccess.GetAccounts.ID IS NOT NULL)
 crit = '(Customer.Accounts.ID = DVARS.ID)';
ELSE IF (AccountAccess.GetAccounts.LastName IS NOT NULL)
BEGIN
 IF (AccountAccess.GetAccounts.LastName == '%')
   ERROR "Last name cannot be %";
 ELSE IF (LOCATE('%', AccountAccess.GetAccounts.LastName) < 0)
   crit = '(Customer.Accounts.Last = DVARS.LastName)';
 ELSE
   crit = '(Customer.Accounts.Last LIKE DVARS.LastName)';
 IF (AccountAccess.GetAccounts.bday IS NOT NULL)
   crit = '(' || crit || ' and (Customer.Accounts.Birthdate = DVARS.BirthDay))';
END
ELSE
 ERROR "ID or LastName must be specified.";

EXECUTE IMMEDIATE 'SELECT ID, First || " " || Last AS Name, Birthdate FROM Customer.Accounts WHERE ' || crit USING ID=AccountAccess.GetAccounts.ID, LastName=AccountAccess.GetAccounts.LastName, BirthDay=AccountAccess.GetAccounts.Bday;
...
----

.Dynamic SQL limitations and workarounds

The use of the dynamic SQL command results in an assignment statement that requires the use of a temporary table.

[source,sql]
.*Example assignment*
----
EXECUTE IMMEDIATE <expression> AS x string INTO #temp;
DECLARE string VARIABLES.RESULT = (SELECT x FROM #temp);
----

The construction of appropriate criteria will be cumbersome if parts of the criteria are not present. 
For example if `criteria` were already NULL, then the following example results in `criteria` remaining NULL.

[source,sql]
.*Example: Dangerous NULL handling*
----
...
criteria = '(' || criteria || ' and (Customer.Accounts.Birthdate = DVARS.BirthDay))';
----

It is best to ensure that the criteria is not NULL prior its usage. 
If this is not possible, a you can specify a default, as shown in the following example.

[source,sql]
.*Example: NULL handling*
----
...
criteria = '(' || nvl(criteria, '(1 = 1)') || ' and (Customer.Accounts.Birthdate = DVARS.BirthDay))';
----

If the dynamic SQL is an `UPDATE`, `DELETE`, or `INSERT` command, the rowcount of the statement can be obtained from the rowcount variable.

[source,sql]
.*Example: AS and INTO clauses*
----
/* Execute an update */
EXECUTE IMMEDIATE <expression>;
----
