Introduction to PL/SQL (SQL Statements)

SQL Statements in PL/SQL: PL/SQL supports data manipulation language (DML) and transaction control language (COMMIT, ROLLBACK, SAVEPOINT). You can use DML commands to modify the data in a database table. PL/SQL dose not directly support DDL statements, such as – CREATE TABLE, ALTER TABLE, DROP TABLE. These statements are dynamic SQL statements. Dynamic statements are built as character string at run time and can contain placeholders and parameters. Therefore, you can use dynamic SQL to execute your DDL statements in PL/SQL. Use the EXECUTE IMMEDIATE statement, which takes the SQL statements as an argument to execute your DDL statements.

SELECT statement: Retrieve data from the database with SELECT statement. In SELECT statement INTO clause is required, and query must return only one row.

Syntax:

SELECT select list ....
INTO {variable_name [, variable_name] ...
     | record_name}
FROM table
 [WHERE condition];

Example:

SET SERVEROUTPUT ON
DECLARE
  sum_sal NUMBER(10,2);
  deptno NUMBER NOT NULL := 60;
BEGIN
  SELECT SUM(salary) -- group function
  INTO sum_sal FROM employees
  WHERE department_id = deptno;
 DBMS_OUTPUT.PUT_LINE ('The sum of the salary is '||sum_sal);
END;
/

If your requirement is to retrieve multiple rows and operate on the data. You can make use of explicit cursor. I discus about later.

Insert Statement:

BEGIN
  INSERT INTO employees
    (employee_id, first_name, last_name, email, salary)
   VALUES (employees_seq.NEXTVAL, 'Ali', 'Asgor', '[email protected]', 4200);
END;
/

Update Statement:

DECLARE
  sal emp.salary%TYPE := 8000;
BEGIN
  UPDATE employees
  SET salary = sal
  WHERE job_id = 'ST_CLERK';
END;
/

Delete Statement:

DECLARE
  dept emp_department_id%TYPE := 10;
BEGIN
  DELETE FROM employees
  WHERE department_id = dept;
END;
/

Merge Statement:

DECLARE
  empno emp.employee_id%TYPE := 100;
BEGIN
  MERGE INTO copy_emp c
    USING emp e
    ON (e.employee_id = c.employee_id)
  WHEN MATCHED THEN
   UPDATE SET
    c.first_name = e.first_name,
    c.last_name  = e.last_name,
    .........
    c.manager_id = e.manager_id
  WHEN NOT MATCHED THEN
   INSERT VALUES (e.employee_id, e.first_name, ...... e.manager_id);
END;
/

SQL Cursor: A cursor is a pointer to the private memory area allocated by the oracle server. There are two type of cursor one is Implicit that’s create and manage internally by the oracle server, another is Explicit cursor that’s explicitly declared by the programer.

SQL Cursor attribute:

Attribute	Description
SQL%FOUND	Boolean Attribute that evaluates to TRUE if the most resent SQL statement returned at least one row.
SQL%NOTFOUND	Boolean Attribute that evaluates to TRUE if the most recent SQL statement did not return even one row.
SQL%ROWCOUNT	An integer value that represents the number or rows affected by the most recent SQL statement

Example:

VARIABLE rows_deleted VARCHAR2(30);
DECLARE
  empno emp.employee_id%TYPE := 105;
BEGIN
  DELETE FROM employees
  WHERE employee_id = empno;
 :rows_deleted := (SQL%ROWCOUNT || 'rows deleted');
END;
/
PRINT rows_deleted

SQL functions: Most of the function are valid in PL/SQL expression. But the following functions are not available in procedural statements.

DECODE
Group functions: AVG, MIN, MAX, COUNT, SUM, STDDEV and VARIANCE. Group functions apply to groups of a rows in a table and therefore are available only in SQL statements in a PL/SQL block.

continue…..