Help with the cartesian product in SQL?
Your definition is right... a query that takes all of the results from one table and matches each row up with every row from the other table. 99%+ of all cartesian (cross) joins are unintentional, where someone forgot a join. If you do not specify the common attribute, you get wrong results -- less or none if you join on a field that has no matches, or more if you omit a join that was necessary. Consider table A: Chris Missouri Melissa West Virginia Erin Texas Brandon West Virginia Adam Texas Kyle Texas And table B: Texas Austin West Virginia Charleston Missouri Springfield A query where you join on the common field (state) would look like this: select * from A join B on A.state = B.state or select * from A, B where a.state = b.state Which would give you six records, essentially supplementing table A with the city. A cartesian product, which omits the join: select * from A, B -- or A cross join B would give you 18 records, each of the names from table A, three records for each, one with each record from table B: Chris Missouri Texas Austin Chris Missouri West Virgina Charleston Chris Missouri Missouri Springfield Melissa West Virginia Texas Austin Melissa West Virginia West Virgina Charleston Melissa West Virginia Missouri Springfield and so on... which is probably not what you meant. There are legitimate uses for cross joins, but in general you only see them when they weren't intended.
What is the difference between a Cartesian product and a join in DBMS?
Cartesian product means you will have rows with each record from one table matched with all rows of second table. i.e if you have 5 rows in first and 5 rows in second table you will get 5x5=25 rows with Cartesian product. If you will use join, you will have keys which will be able to join rows from first table with rows of second table and depending on your relation, number of rows may vary.
How to do a cartesian product on these tables (SQL)?
The keyword Cross Join returns the Cartesian product of rows from tables in the join. It combines each row from the first table with each row from the second table. Example of a implicit cross join (the system percieves it as a cross join) is: Select * from employee, department; example of a explicit cross join (by explictly using the keyword) is: Select * from employee CROSS JOIN department; You can use any of the above ways to get the Cartesian product in MS SQL. Good Luck.!
SQL Query help....?
SELECT T_patient.name, Max(T_appointment.Date) AS LastDate FROM T_patient INNER JOIN T_appointment ON T_patient.name = T_appointment.name GROUP BY T_patient.name; This is a basic join of two tables using the patients name in a one to many relationship. You will have to use your tables primary and foreign keys whatever they may be.(SSN, recordID ect). for example: WHERE T_patient.(primary key like SSN) = T_appointment.(foreign key like SSN) Also you cannot use the * to return all fields, rather you will have to add each field name from the patients record table. I have only added a single field here for clarity. This SQL statement will return a single record based upon the matching of the max date value in the T_appointment table.
What are some practical uses of SQL Cartesian Joins?
I came across a problem which is solvable using a cartesian join.Suppose you have a table with employee id, name, department and salary for the employee.And you want to print the employees for the department where the average salary of a department is greater than one fourth the total salary paid by the whole company.You can write a sql which looks like the followingSELECT * FROM (EMPID, EMP_NAME, DEPT, EMPSAL, AVG(EMP_SAL) OVER(PARTITION BY DEPT) AS AVG_SAL_DEP , TOTAL_SAL FROM EMPLOYEE, (SELECT SUM(EMP_SAL) AS TOTAL_SAL FROM EMPLOYEE)) WHERE AVG_SAL_DEP > 1/4*TOTAL_SAL;This query effectively uses the cartesian join to map each row of the original employee table with the sum of salary (total salary paid by the company to its employees) and uses analytical Avg function to get the result.
Why should Cartesian joins be avoided?
You’ll get a “Cartesian join” if you have a SQL query that has JOIN tables but no join predicates, ieselect f.*, g.* from tab1 f join tab2 g;orselect f.*, g.* from tab1 f, tab2 g;What you’ll end up with a result that has every row in tab1 glued to every row in tab2. If tab1 has 3 rows and tab2 has 4 rows, you’ll fetch 12 projection rows.While I’ve found a use for this on occasion - primarily when building work tables for reports - this is almost never what you want in an actual query on real-life tables that have an interestingly large number of rows in them.So be sure your joins have predicates. Your DBA and network administrator will be grateful :)
How do you join 3 tables in SQL with no duplicates?
Good answer: Use the correct Primary and Foreign Keys to join the tables. This will avoid the dreaded Cartesian Product, with many times the desired number of returned rows most of which are duplicates.Bad answer: Carefully limit the columns in your result set and use the keyword DISTINCT. This will still extract the full Cartesian Product, but will return only unique rows based on the columns in your SELECT statement. Your query will still run slowly. Furthermore because either the tables don't contain, or you're not using, the appropriate Primary and Foreign Keys you need to be very sure of the data in all of those tables to be confident of returning the correct results in every case.So use the good answer, but keep the bad answer on your toolbelt for times when you really need it.If you're unsure about the Keys I refer to above, may I recommend the excellent book “SQL For Dummies”, with no disrespect intended. Good stuff in there.
Gantt chart using access and crystal reports 10?
that's sounds pretty confusing. good luck with it!