4  Using Embedded SQL

Output versus Input Host Variables

Depending on how they are used, host variables are called output or input host variables. Host variables in the INTO clause of a SELECT or FETCH statement are called output host variables because they hold column values output by Oracle. Oracle assigns the column values to corresponding output host variables in the INTO clause.

All other host variables in a SQL statement are called input host variables because your program inputs their values to Oracle. For example, you use input host variables in the VALUES clause of an INSERT statement and in the SET clause of an UPDATE statement. They are also used in the WHERE, HAVING, and FOR clauses. In fact, input host variables can appear in a SQL statement wherever a value or expression is allowed.

In an ORDER BY clause, you can use a host variable, but it is treated as a constant or literal, and hence the contents of the host variable have no effect. For example, the SQL statement

EXEC SQL SELECT ename, empno INTO :name, :number
 FROM emp
 ORDER BY :ord;

appears to contain an input host variable, ord. However, the host variable in this case is treated as a constant, and regardless of the value of ord, no ordering is done.

You cannot use input host variables to supply SQL keywords or the names of database objects. Thus, you cannot use input host variables in data definition statements (sometimes called DDL) such as ALTER, CREATE, and DROP. In the following example, the DROP TABLE statement is invalid:

EXEC SQL BEGIN DECLARE SECTION; 
 table_name CHARACTER(30); 
EXEC SQL END DECLARE SECTION; 
display 'Table name? '; 
read table_name; 
EXEC SQL DROP TABLE :table_name; -- host variable not allowed

Before Oracle executes a SQL statement containing input host variables, your program must assign values to them. Consider the following example:

EXEC SQL BEGIN DECLARE SECTION; 
 emp_number INTEGER; 
 emp_name CHARACTER(20); 
EXEC SQL END DECLARE SECTION; 
-- get values for input host variables 
display 'Employee number? '; 
read emp_number; 
display 'Employee name? '; 
read emp_name; 
EXEC SQL INSERT INTO EMP (EMPNO, ENAME) 
 VALUES (:emp_number, :emp_name);

Notice that the input host variables in the VALUES clause of the INSERT statement are prefixed with colons.

Input Variables

For input host variables, the values your program can assign to an indicator variable have the following meanings:

• -1: Oracle will assign a null to the column, ignoring the value of the host variable.

• >= 0: Oracle will assigns the value of the host variable to the column.

Output Variables

For output host variables, the values Oracle can assign to an indicator variable have the following meanings:

• -2: Oracle assigned a truncated column value to the host variable, but could not assign the original length of the column value to the indicator variable because the number was too large.

• -1: The column value is null, so the value of the host variable is indeterminate.

• 0: Oracle assigned an intact column value to the host variable.

• > 0: Oracle assigned a truncated column value to the host variable, assigned the original column length (expressed in characters, instead of bytes, for multibyte Globalization Support host variables) to the indicator variable, and set SQLCODE in the SQLCA to zero.

Remember, an indicator variable must be defined in the Declare Section as a 2-byte integer and, in SQL statements, must be prefixed with a colon and appended to its host variable (unless you use the keyword INDICATOR).

Inserting Nulls

You can use indicator variables to insert nulls. Before the insert, for each column you want to be null, set the appropriate indicator variable to -1, as shown in the following example:

set ind_comm = -1; 
EXEC SQL INSERT INTO EMP (EMPNO, COMM) 
 VALUES (:emp_number, :commission:ind_comm);

The indicator variable ind_comm specifies that a null is to be stored in the COMM column.

You can hardcode the null instead, as follows:

EXEC SQL INSERT INTO EMP (EMPNO, COMM) 
 VALUES (:emp_number, NULL);

While this is less flexible, it might be more readable.

Typically, you insert nulls conditionally, as the next example shows:

display 'Enter employee number or 0 if not available: '; 
read emp_number; 
IF emp_number = 0 THEN 
 set ind_empnum = -1; ELSE 
 set ind_empnum = 0; 
ENDIF; 
EXEC SQL INSERT INTO EMP (EMPNO, SAL) 
 VALUES (:emp_number:ind_empnum, :salary);

Handling Returned Nulls

You can also use indicator variables to manipulate returned nulls, as the following example shows:

EXEC SQL SELECT ENAME, SAL, COMM 
 INTO :emp_name, :salary, :commission:ind_comm 
 FROM EMP 
 WHERE EMPNO = :emp_number; 
IF ind_comm = -1 THEN 
 set pay = salary; -- commission is null; ignore it 
ELSE 
 set pay = salary + commission; 
ENDIF;

Fetching Nulls

When DBMS=NATIVE, V7, or V8, if you select or fetch nulls into a host variable that lacks an indicator variable, Oracle issues the following error message:

ORA-01405: fetched column value is NULL

Testing for Nulls

You can use indicator variables in the WHERE clause to test for nulls, as the following example shows:

EXEC SQL SELECT ENAME, SAL 
 INTO :emp_name, :salary 
 FROM EMP 
 WHERE :commission:ind_comm IS NULL ...

However, you cannot use a relational operator to compare nulls with each other or with other values. For example, the following SELECT statement fails if the COMM column contains one or more nulls:

EXEC SQL SELECT ENAME, SAL 
 INTO :emp_name, :salary 
 FROM EMP 
 WHERE COMM = :commission:ind_comm;

The next example shows how to compare values for equality when some of them might be nulls:

EXEC SQL SELECT ENAME, SAL 
 INTO :emp_name, :salary 
 FROM EMP 
 WHERE (COMM = :commission) OR ((COMM IS NULL) AND 
 (:commission:ind_comm IS NULL));

Fetching Truncated Values

If you select or fetch a truncated column value into a host variable that lacks an indicator variable, no error is generated.

Selecting Rows

Querying the database is a common SQL operation. To issue a query you use the SELECT statement. In the following example, you query the EMP table:

EXEC SQL SELECT ENAME, JOB, SAL + 2000 
 INTO :emp_name, :job_title, :salary 
 FROM EMP 
 WHERE EMPNO = :emp_number;

The column names and expressions following the keyword SELECT make up the select list. The select list in our example contains three items. Under the conditions specified in the WHERE clause (and following clauses, if present), Oracle returns column values to the host variables in the INTO clause. The number of items in the select list should equal the number of host variables in the INTO clause, so there is a place to store every returned value.

In the simplest case, when a query returns one row, its form is that shown in the last example (in which EMPNO is a unique key). However, if a query can return more than one row, you must fetch the rows using a cursor or select them into a host array.

If you write a query to return only one row but it might actually return several rows, the result depends on how you specify the option SELECT_ERROR. When SELECT_ERROR=YES (the default), Oracle issues the following error message if more than one row is returned:

ORA-01422: exact fetch returns more than requested number of rows

When SELECT_ERROR=NO, a row is returned and Oracle generates no error.

Available Clauses

You can use all of the following standard SQL clauses in your SELECT statements: INTO, FROM, WHERE, CONNECT BY, START WITH, GROUP BY, HAVING, ORDER BY, and FOR UPDATE OF.

Inserting Rows

You use the INSERT statement to add rows to a table or view. In the following example, you add a row to the EMP table:

EXEC SQL INSERT INTO EMP (EMPNO, ENAME, SAL, DEPTNO) 
 VALUES (:emp_number, :emp_name, :salary, :dept_number);

Each column you specify in the column list must belong to the table named in the INTO clause. The VALUES clause specifies the row of values to be inserted. The values can be those of constants, host variables, SQL expressions, or pseudocolumns, such as USER and SYSDATE.

The number of values in the VALUES clause must equal the number of names in the column list. However, you can omit the column list if the VALUES clause contains a value for each column in the table in the same order they were defined by CREATE TABLE.

Using Subqueries

A subquery is a nested SELECT statement. Subqueries let you conduct multipart searches. They can be used to

• supply values for comparison in the WHERE, HAVING, and START WITH clauses of SELECT, UPDATE, and DELETE statements

• define the set of rows to be inserted by a CREATE TABLE or INSERT statement

• define values for the SET clause of an UPDATE statement

For example, to copy rows from one table to another, replace the VALUES clause in an INSERT statement with a subquery, as follows:

EXEC SQL INSERT INTO EMP2 (EMPNO, ENAME, SAL, DEPTNO) 
 SELECT EMPNO, ENAME, SAL, DEPTNO FROM EMP 
 WHERE JOB = :job_title;

Notice how the INSERT statement uses the subquery to obtain intermediate results.

Updating Rows

You use the UPDATE statement to change the values of specified columns in a table or view. In the following example, you update the SAL and COMM columns in the EMP table:

EXEC SQL UPDATE EMP 
 SET SAL = :salary, COMM = :commission 
 WHERE EMPNO = :emp_number;

You can use the optional WHERE clause to specify the conditions under which rows are updated. See "Using the WHERE Clause".

The SET clause lists the names of one or more columns for which you must provide values. You can use a subquery to provide the values, as the following example shows:

EXEC SQL UPDATE EMP 
 SET SAL = (SELECT AVG(SAL)*1.1 FROM EMP WHERE DEPTNO = 20) 
 WHERE EMPNO = :emp_number;

Deleting Rows

You use the DELETE statement to remove rows from a table or view. In the following example, you delete all employees in a given department from the EMP table:

EXEC SQL DELETE FROM EMP 
 WHERE DEPTNO = :dept_number;

You can use the optional WHERE clause to specify the condition under which rows are deleted.

Using the WHERE Clause

You use the WHERE clause to select, update, or delete only those rows in a table or view that meet your search condition. The WHERE-clause search condition is a Boolean expression, which can include scalar host variables, host arrays (not in SELECT statements), and subqueries.

If you omit the WHERE clause, all rows in the table or view are processed. If you omit the WHERE clause in an UPDATE or DELETE statement, Oracle sets SQLWARN(5) in the SQLCA to 'W' to warn that all rows were processed.

Declaring a Cursor

You use the DECLARE statement to define a cursor by giving it a name and associating it with a query, as the following example shows:

EXEC SQL DECLARE emp_cursor CURSOR FOR 
 SELECT ENAME, EMPNO, SAL 
 FROM EMP 
 WHERE DEPTNO = :dept_number;

The cursor name is an identifier used by the precompiler, not a host or program variable, and should not be defined in the Declare Section. Therefore, cursor names cannot be passed from one precompilation unit to another. Also, cursor names cannot be hyphenated. They can be any length, but only the first 31 characters are significant. For ANSI compatibility, use cursor names no longer than 18 characters.

The SELECT statement associated with the cursor cannot include an INTO clause. Rather, the INTO clause and list of output host variables are part of the FETCH statement.

Because it is declarative, the DECLARE statement must physically (not just logically) precede all other SQL statements referencing the cursor. That is, forward references to the cursor are not allowed. In the following example, the OPEN statement is misplaced:

EXEC SQL OPEN emp_cursor; -- misplaced OPEN statement
EXEC SQL DECLARE emp_cursor CURSOR FOR 
 SELECT ENAME, EMPNO, SAL 
 FROM EMP 
 WHERE ENAME = :emp_name;

The cursor control statements must all occur within the same precompiled unit. For example, you cannot declare a cursor in file A, then open it in file B.

Your host program can declare as many cursors as it needs. However, in a given file, every DECLARE statement must be unique. That is, you cannot declare two cursors with the same name in one precompilation unit, even across blocks or procedures, because the scope of a cursor is global within a file. If you will be using many cursors, you might want to specify the MAXOPENCURSORS option. For more information, see "MAXOPENCURSORS".

Opening a Cursor

Use the OPEN statement to execute the query and identify the active set. In the following example, a cursor named emp_cursor is opened.

EXEC SQL OPEN emp_cursor;

OPEN positions the cursor just before the first row of the active set. It also zeroes the rows-processed count kept by SQLERRD(3) in the SQLCA. However, none of the rows is actually retrieved at this point. That will be done by the FETCH statement.

After you open a cursor, the query's input host variables are not reexamined until you reopen the cursor. Thus, the active set does not change. To change the active set, you must reopen the cursor.

Generally, you should close a cursor before reopening it. However, if you specify MODE=ORACLE (the default), you need not close a cursor before reopening it. This can boost performance; for details, see Appendix C, "Performance Tuning"

The amount of work done by OPEN depends on the values of three precompiler options: HOLD_CURSOR, RELEASE_CURSOR, and MAXOPENCURSORS. For more information, see "Using the Precompiler Options".

Fetching from a Cursor

You use the FETCH statement to retrieve rows from the active set and specify the output host variables that will contain the results. Recall that the SELECT statement associated with the cursor cannot include an INTO clause. Rather, the INTO clause and list of output host variables are part of the FETCH statement. In the following example, you fetch into three host variables:

EXEC SQL FETCH emp_cursor 
 INTO :emp_name, :emp_number, :salary;

The cursor must have been previously declared and opened. The first time you execute FETCH, the cursor moves from before the first row in the active set to the first row. This row becomes the current row. Each subsequent execution of FETCH advances the cursor to the next row in the active set, changing the current row. The cursor can only move forward in the active set. To return to a row that has already been fetched, you must reopen the cursor, then begin again at the first row of the active set.

If you want to change the active set, you must assign new values to the input host variables in the query associated with the cursor, then reopen the cursor. When MODE={ANSI | ANSI14 | ANSI13}, you must close the cursor before reopening it.

As the next example shows, you can fetch from the same cursor using different sets of output host variables. However, corresponding host variables in the INTO clause of each FETCH statement must have the same datatype.

EXEC SQL DECLARE emp_cursor CURSOR FOR 
 SELECT ENAME, SAL FROM EMP WHERE DEPTNO = 20; 
... 
EXEC SQL OPEN emp_cursor; 
EXEC SQL WHENEVER NOT FOUND DO ... 
LOOP 
 EXEC SQL FETCH emp_cursor INTO :emp_name1, :salary1; 
 EXEC SQL FETCH emp_cursor INTO :emp_name2, :salary2; 
 EXEC SQL FETCH emp_cursor INTO :emp_name3, :salary3; 
 ... 
ENDLOOP;

If the active set is empty or contains no more rows, FETCH returns the "no data found" Oracle warning code to SQLCODE in the SQLCA (or when MODE=ANSI, to the status variable SQLSTATE). The status of the output host variables is indeterminate. (In a typical program, the WHENEVER NOT FOUND statement detects this error.) To reuse the cursor, you must reopen it.

Closing a Cursor

When finished fetching rows from the active set, you close the cursor to free the resources, such as storage, acquired by opening the cursor. When a cursor is closed, parse locks are released. What resources are freed depends on how you specify the options HOLD_CURSOR and RELEASE_CURSOR. In the following example, you close the cursor named emp_cursor:

EXEC SQL CLOSE emp_cursor;

You cannot fetch from a closed cursor because its active set becomes undefined. If necessary, you can reopen a cursor (with new values for the input host variables, for example).

When MODE={ANSI13|ORACLE}, issuing a commit or rollback closes cursors referenced in a CURRENT OF clause. Other cursors are unaffected by a commit or rollback and if open, remain open. However, when MODE={ANSI|ANSI14}, issuing a commit or rollback closes all explicit cursors.

Using the CURRENT OF Clause

You use the CURRENT OF cursor_name clause in a DELETE or UPDATE statement to refer to the latest row fetched from the named cursor. The cursor must be open and positioned on a row. If no fetch has been done or if the cursor is not open, the CURRENT OF clause results in an error and processes no rows.

The FOR UPDATE OF clause is optional when you declare a cursor that is referenced in the CURRENT OF clause of an UPDATE or DELETE statement. The CURRENT OF clause signals the precompiler to add a FOR UPDATE clause if necessary. For more information, see "Using the FOR UPDATE OF Clause".

In the following example, you use the CURRENT OF clause to refer to the latest row fetched from a cursor named emp_cursor:

EXEC SQL DECLARE emp_cursor CURSOR FOR 
 SELECT ENAME, SAL FROM EMP WHERE JOB = 'CLERK' 
 FOR UPDATE OF SAL; 
... 
EXEC SQL OPEN emp_cursor; 
EXEC SQL WHENEVER NOT FOUND DO ... 
LOOP 
 EXEC SQL FETCH emp_cursor INTO :emp_name, :salary; 
 ... 
 EXEC SQL UPDATE EMP SET SAL = :new_salary 
 WHERE CURRENT OF emp_cursor; 
ENDLOOP;

Restrictions

An explicit FOR UPDATE OF or an implicit FOR UPDATE acquires exclusive row locks. All rows are locked at the open, not as they are fetched, and are released when you commit or rollback. If you try to fetch from a FOR UPDATE cursor after a commit, Oracle generates the following error:

ORA-01002: fetch out of sequence

You cannot use host arrays with the CURRENT OF clause. For an alternative, see "Mimicking the CURRENT OF Clause". Also, you cannot reference multiple tables in an associated FOR UPDATE OF clause, which means that you cannot do joins with the CURRENT OF clause. Finally, you cannot use the CURRENT OF clause in dynamic SQL.

A Typical Sequence of Statements

The following example shows the typical sequence of cursor control statements in an application program:

-- Define a cursor.
EXEC SQL DECLARE emp_cursor CURSOR FOR 
 SELECT ENAME, JOB FROM EMP 
 WHERE EMPNO = :emp_number 
 FOR UPDATE OF JOB; 

-- Open the cursor and identify the active set.
EXEC SQL OPEN emp_cursor; 
-- Exit if the last row was already fetched.
EXEC SQL WHENEVER NOT FOUND DO no_more; 

-- Fetch and process data in a loop.
LOOP 
 EXEC SQL FETCH emp_cursor INTO :emp_name, :job_title; 
 -- host-language statements that operate on the fetched data 
 EXEC SQL UPDATE EMP 
 SET JOB = :new_job_title 
 WHERE CURRENT OF emp_cursor; 
ENDLOOP; 
... 
ROUTINE no_more
BEGIN
-- Disable the cursor.
 EXEC SQL CLOSE emp_cursor; 
 EXEC SQL COMMIT WORK RELEASE; 
 exit program; 
END no_more;

A Complete Example

The following program illustrates the use of a cursor and the FETCH statement. The program prompts for a department number, then displays the names of all employees in that department.

All fetches except the final one return a row and, if no errors were detected during the fetch, a success status code. The final fetch fails and returns the "no data found" Oracle warning code to SQLCODE in the SQLCA. The cumulative number of rows actually fetched is found in SQLERRD(3) in the SQLCA.

-- declare host variables 
EXEC SQL BEGIN DECLARE SECTION; 
 username CHARACTER(20); 
 password CHARACTER(20); 
 emp_name CHARACTER(10); 
 dept_number INTEGER; 
EXEC SQL END DECLARE SECTION; 
-- copy in the SQL Communications Area 
EXEC SQL INCLUDE SQLCA; 

display 'Username? '; 
read username; 
display 'Password? '; 
read password; 

-- handle processing errors 
EXEC SQL WHENEVER SQLERROR DO sql_error; 

-- log on to Oracle 
EXEC SQL CONNECT :username IDENTIFIED BY :password; 
display 'Connected to Oracle'; 

-- declare a cursor 
EXEC SQL DECLARE emp_cursor CURSOR FOR 
 SELECT ENAME FROM EMP WHERE DEPTNO = :dept_number; 

display 'Department number? '; 
read dept_number; 

-- open the cursor and identify the active set 
EXEC SQL OPEN emp_cursor; 

-- exit if the last row was already fetched 
EXEC SQL WHENEVER NOT FOUND DO no_more; 

display 'Employee Name'; 
display '-------------'; 

-- fetch and process data in a loop 
LOOP 
 EXEC SQL FETCH emp_cursor INTO :emp_name; display emp_name; 
ENDLOOP;
ROUTINE no_more
BEGIN
 EXEC SQL CLOSE emp_cursor; 
 EXEC SQL COMMIT WORK RELEASE; 
 display 'End of program'; 
 exit program; 
END no_more;

ROUTINE sql_error
BEGIN
 EXEC SQL WHENEVER SQLERROR CONTINUE; 
 EXEC SQL ROLLBACK WORK RELEASE; 
 display 'Processing error'; 
 exit program with an error; 
END sql_error;

Declaring a Cursor Variable

How you declare a cursor variable is dependent on your host language. For instructions about declaring a cursor variable, see your host-language supplement.

Allocating a Cursor Variable

You use the ALLOCATE statement to allocate memory for the cursor variable. The syntax follows:

EXEC SQL ALLOCATE <cursor_variable>;

Opening a Cursor Variable

You use the OPEN ... FOR statement to associate a cursor variable with a multi-row query, execute the query, and identify the active set. The syntax follows:

EXEC SQL OPEN <cursor_variable> FOR <select_statement>;

The SELECT statement can reference input host variables and PL/SQL variables, parameters, and functions but cannot be FOR UPDATE. In the following example, you open a cursor variable named emp_cv:

EXEC SQL OPEN emp_cv FOR SELECT * FROM EMP;

You must open a cursor variable on the server side. You do that by passing it as an input host variable to an anonymous PL/SQL block. At run time, the block is sent to the Oracle Server for execution. In the following example, you declare and initialize a cursor variable, then pass it to a PL/SQL block, which opens the cursor variable:

EXEC SQL BEGIN DECLARE SECTION;
 ...
-- declare cursor variable
 emp_cur SQL_CURSOR;
EXEC SQL END DECLARE SECTION;
 
-- initialize cursor variable
EXEC SQL ALLOCATE :emp_cur;
 
EXEC SQL EXECUTE
 -- pass cursor variable to PL/SQL block
 BEGIN
 -- open cursor variable
 OPEN :emp_cur FOR SELECT * FROM EMP;
 END;
END-EXEC;

Generally, you pass a cursor variable to PL/SQL by calling a stored procedure that declares a cursor variable as one of its formal parameters. For example, the following packaged procedure opens a cursor variable named emp_cv:

CREATE PACKAGE emp_data AS
 -- define REF CURSOR type
 TYPE EmpCurTyp IS REF CURSOR RETURN emp%ROWTYPE;
 -- declare formal paramter of that type
 PROCEDURE open_emp_cv (emp_cv IN OUT EmpCurTyp);
END emp_data;
 
CREATE PACKAGE BODY emp_data AS
 PROCEDURE open_emp_cv (emp_cv IN OUT EmpCurTyp) IS
 BEGIN
 -- open cursor variable
 OPEN emp_cv FOR SELECT * FROM emp;
 END open_emp_cv;
END emp_data;

You can call the procedure from any application, as follows:

EXEC SQL EXECUTE
 BEGIN 
 emp_data.open_emp_cv(:emp_cur); 
 END; 
END-EXEC;

Fetching from a Cursor Variable

After opening a cursor variable for a multi-row query, you use the FETCH statement to retrieve rows from the active set one at a time. The syntax follows:

EXEC SQL FETCH cursor_variable_name 
 INTO {record_name | variable_name[, variable_name, ...]};

Each column value returned by the cursor variable is assigned to a corresponding field or variable in the INTO clause, providing their datatypes are compatible.

The FETCH statement must be executed on the client side. In the following example, you fetch rows into a host record named emp_rec:

-- exit loop when done fetching
EXEC SQL WHENEVER NOT FOUND DO no_more;
LOOP
 -- fetch row into record
 EXEC SQL FETCH :emp_cur INTO :emp_rec; 
 -- process the data
ENDLOOP;

Closing a Cursor Variable

You use the CLOSE statement to close a cursor variable, at which point its active set becomes undefined. The syntax follows:

EXEC SQL CLOSE cursor_variable_name;

The CLOSE statement can be executed on the client side or the server side. In the following example, when the last row is processed, you close the cursor variable emp_cur:

-- close cursor variable
EXEC SQL CLOSE :emp_cur;