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BACS 287

BACS 287. Structured Query Language 1. Visual Basic Table Access. Visual Basic provides 2 mechanisms to access data in tables: Record-at-a-time access MoveFirst, MoveLast,MoveNext, MovePrevious Set-at-a-time access Select, Insert, Update, Delete

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BACS 287

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  1. BACS 287 Structured Query Language 1 BACS 287

  2. Visual Basic Table Access • Visual Basic provides 2 mechanisms to access data in tables: • Record-at-a-time access • MoveFirst, MoveLast,MoveNext, MovePrevious • Set-at-a-time access • Select, Insert, Update, Delete • Both methods can accomplish the same thing, but set-at-a-time is usually more efficient. BACS 287

  3. Visual Basic Table Access • Set-at-a-time access means that you do not have to explicitly manipulate the record pointer or perform a loop. • This creates a non-procedural environment were you describe what the solution looks like, not how to do it. • The most popular set-at-a-time language is called Structured Query Language. BACS 287

  4. Structured Query Language • Structured Query Language (SQL) is a 4th generation language designed to work with relational sets. • Commands exist to create and load tables, select subsets of tables, and modify existing tables. • SQL is not a full programming language. It is intended to be embedded in another, more traditional, language. BACS 287

  5. Structured Query Language • SQL commands are divided into 3 primary groups: • Data Definition Language commands • Data Manipulation Language commands • Data Control Language commands • Visual Basic supports the first 2 categories very well. It does not have complete support for the 3rd group. BACS 287

  6. Data Manipulation Language • Visual Basic DML commands include: • SELECT Retrieve data from a database, create copies of tables, specify tuples for updating • INSERT Add data to tables • UPDATE Change existing data in tables • DELETE Remove data from tables BACS 287

  7. Generic Select Statement • By far, the most commonly used DML statement is the SELECT. It combines a range of functionality into one complex command. This is the generic format. SELECT {field-list | * | DISTINCTROW field} FROM table-list WHERE expression GROUP BY group-fields HAVING group-expression ORDER BY field-list; BACS 287

  8. Select Clauses • SELECT – The list of “columns” to select from the table. Also allows you to select all (with *), or to remove duplicate rows (with DISTINCTROW). • FROM - A required clause that lists the tables that the select works on. You can define "alias" names with this clause to speed up query input and to allow recursive "self-joins". • WHERE - An optional clause that selects rows that meet the stated condition. A "sub-select" can appear as the expression of a where clause. This is called a "nested select". BACS 287

  9. Select Clauses • GROUP BY - An optional clause that groups rows according to the values in one or more columns and sorts the results in ascending order (unless otherwise specified). The duplicate rows are not eliminated, rather they are consolidated into one row. This is similar to a control break in traditional programming. • HAVING - An optional clause that is used with GROUP BY. It selects from the rows that result from applying the GROUP BY clause. This works the same as the WHERE clause, except that it only applies to the output of GROUP BY. BACS 287

  10. Select Clauses • ORDER BY - An optional clause that sorts the final result of the SELECT into either ascending or descending order on one or more named columns. • There can be complex interaction between the WHERE, GROUP BY, and HAVING clauses. When all three are present the WHERE is done first, the GROUP BY is done second, and the HAVING is done last. BACS 287

  11. Single Table Select Examples Example 1: Retrieve all information about students SELECT * FROM STUDENT; Example 2: Find the last name, Bear ID, and credits of all students SELECT LAST_NAME, BEAR_NUM, CREDITS FROM STUDENT; BACS 287

  12. Select Examples Example 3: Select all employees from the 'ACCT' department. SELECT * FROM EMPLOYEES WHERE EMP-DEPT = 'ACCT'; Example 4: Show what salary would be if each employee received a 10% raise. SELECT LAST_NAME, SALARY AS CURRENT, SALARY * 1.1 AS PROPOSED FROM EMPLOYEES; BACS 287

  13. Single Table Select Examples Example 5: Find all information about students who are math majors SELECT * FROM STUDENT WHERE MAJOR = 'Math'; Example 6: Find the Bear ID of all History majors SELECT BEAR_NUM FROM STUDENT WHERE MAJOR = 'History'; BACS 287

  14. Enhanced Where Clauses The WHERE clause can be enhanced to be more selective. Operators that can appear in WHERE conditions include: =, <> ,< ,> ,>= ,<= IN BETWEEN...AND... LIKE IS NULL AND, OR, NOT BACS 287

  15. Single Table Select Examples Example 1: Find the student ID of all math majors with more than 30 credit hours. SELECT BEAR_NUM FROM STUDENT WHERE MAJOR = 'Math' AND CREDITS > 30; Example 2: Find the student ID and last name of students with between 30 and 60 hours (inclusive). SELECT BEAR_NUM, LAST_NAME FROM STUDENT WHERE CREDITS BETWEEN 30 AND 60; BACS 287

  16. Single Table Select Examples Example 3: Retrieve the ID of all students who are either a math or an art major. SELECT BEAR_NUM FROM STUDENT WHERE MAJOR IN ('Math','Art'); Example 4: Retrieve the ID and course number of all students without a grade in a class. SELECT BEAR_NUM, COURSENUM FROM ENROLL WHERE GRADE IS NULL; BACS 287

  17. Single Table Select Examples Example 5: List the course number and faculty ID for all math courses. SELECT COURSENUM, FACULTY_ID FROM CLASS WHERE COURSENUM LIKE 'MTH*'; BACS 287

  18. Aggregate Function Select • SQL also allows several aggregate functions to appear in the SELECT line of the SELECT statement. These include: Max, Min, Avg, Sum, Count, StdDev, StdDevP, Var, VarP. Example 1: How many students are there? SELECT COUNT(*) FROM STUDENT; BACS 287

  19. Aggregate Function Select Example 2: Find the number of departments that have faculty in them. SELECT COUNT(DISTINCTROW DEPT) FROM FACULTY; Example 3: Find the average number of credits for students who major in math. SELECT AVG(CREDITS) FROM STUDENT WHERE MAJOR = 'Math'; BACS 287

  20. Ordering the Select Result Example 1: List the names and IDs of all faculty members arranged in alphabetical order. SELECT FACULTY_ID, FACULTY_NAME FROM FACULTY ORDER BY FACULTY_NAME; Example 2: List names and IDs of faculty members. SELECT FACULTY_ID, FACULTY_NAME FROM FACULTY ORDER BY FACULTY_NAME, FACULTY_ID DESC; BACS 287

  21. SQL Join Operation • A JOIN operation is performed when more than one table is specified in the FROM clause. You join two tables if you need information from both. • You must specify the JOIN condition explicitly in SQL. This includes naming the columns the two tables have in common and the comparison operator. BACS 287

  22. SQL Join Examples Example 1: Find the name and courses that each faculty member teaches. SELECT FACULTY.FACNAME, COURSENUM FROM FACULTY, CLASS WHERE FACULTY.FACID = CLASS.FACID; • Note how the table name is appended to the FACNAME field of the SELECT clause. This is called qualification. It is required if the same name is used in 2 tables. BACS 287

  23. SQL Join Examples Example 2: Find the course number and the major of all students taught by the faculty member with ID number 'F110'. (3 table JOIN) SELECT ENROLL.COURSENUM, LAST_NAME, MAJOR FROM CLASS , ENROLL, STUDENT WHERE FACULTY_ID = 'F110' AND CLASS.COURSENUM = ENROLL.COURSENUM AND ENROLL.BEAR_NUM = STUDENT.BEAR_NUM; BACS 287

  24. Views • Views are a way to save your select queries so that you do not have to build them each time you need them. • The view saves the procedure (not the result) for he query. • Views are a “free” form of security BACS 287

  25. Views • Views are used to simplify queries and to provide security. • They are often called "virtual tables" because the table is not stored in the database. Instead, the procedure to derive the view is stored. • The view is generated whenever it is requested, thus it is always up-to-date and does not take up any disk space. BACS 287

  26. Views • You build views by first creating a valid select and then adding one line of code before the select. • Any valid select can fill in the select portion. • In all cases (except for update) views can be used in the same was as select statements. BACS 287

  27. Views Example: Build a view called CLASS_LIST that contains the student IDs and last name for all students in the class 'ART103A'. CREATE VIEW CLASS_LIST AS SELECT BEAR_NUM, LAST_NAME FROM ENROLL, STUDENT WHERE COURSENUM = 'ART103A' AND ENROLL.BEAR_NUM = STUDENT.BEAR_NUM; BACS 287

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