Q&A: Database Programming

  1. What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?

Cursors allow row-by-row processing of the resultsets. Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. See books online for more information. Disadvantages of cursors: Each time you fetch a row from the cursor, it results in a network roundtrip, where as a normal SELECT query makes only one roundtrip, however large the resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Further, there are restrictions on the SELECT statements that can be used with some types of cursors. Most of the times, set based operations can be used instead of cursors. Here is an example: If you have to give a flat hike to your employees using the following criteria:

 

Salary between 30000 and 40000 — 5000 hike

Salary between 40000 and 55000 — 7000 hike

Salary between 55000 and 65000 — 9000 hike.

 

In this situation many developers tend to use a cursor, determine each employee’s salary and update his salary according to the above formula. But the same can be achieved by multiple update statements or can be combined in a single UPDATE statement as shown below:

UPDATE tbl_emp SET salary = CASE WHEN salary BETWEEN 30000 AND 40000 THEN salary + 5000 WHEN salary BETWEEN 40000 AND 55000 THEN salary + 7000 WHEN salary BETWEEN 55000 AND 65000 THEN salary + 10000 END

Another situation in which developers tend to use cursors: You need to call a stored procedure when a column in a particular row meets certain condition. You don’t have to use cursors for this. This can be achieved using WHILE loop, as long as there is a unique key to identify each row.

 

2. Write down the general syntax for a SELECT statements covering all the options.

Here’s the basic syntax:

SELECT select_list [INTO new_table_] FROM table_source [WHERE search_condition] [GROUP BY group_by_expression] [HAVING search_condition] [ORDER BY order_expression [ASC | DESC] ]

3. What is a join and explain different types of joins.

Joins are used in queries to explain how different tables are related. Joins also let you select data from a table depending upon data from another table. Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS. For more information see pages from books online titled: “Join Fundamentals” and “Using Joins”.

 

Join conditions can be specified in either the FROM or WHERE clauses; specifying them in the FROM clause is recommended. WHERE and HAVING clauses can also contain search conditions to further filter the rows selected by the join conditions. Joins can be categorized as:

  • Inner joins (the typical join operation, which uses some comparison operator like = or <>).

These include equi-joins and natural joins. Inner joins use a comparison operator to match rows from two tables based on the values in common columns from each table. For example, retrieving all rows where the student identification number is the same in both the students and courses tables.

  • Outer joins. Outer joins can be a left, a right, or full outer join.

Outer joins are specified with one of the following sets of keywords when they are specified in the FROM clause: LEFT JOIN or LEFT OUTER JOIN. The result set of a left outer join includes all the rows from the left table specified in the LEFT OUTER clause, not just the ones in which the joined columns match. When a row in the left table has no matching rows in the right table, the associated result set row contains null values for all select list columns coming from the right table.

  • RIGHT JOIN or RIGHT OUTER JOIN

A right outer join is the reverse of a left outer join. All rows from the right table are returned. Null values are returned for the left table any time a right table row has no matching row in the left table.

  • FULL JOIN or FULL OUTER JOIN

A full outer join returns all rows in both the left and right tables. Any time a row has no match in the other table, the select list columns from the other table contain null values. When there is a match between the tables, the entire result set row contains data values from the base tables.

  • Cross joins

Cross joins return all rows from the left table. Each row from the left table is combined with all rows from the right table. Cross joins are also called Cartesian products.

For example, here is an inner join retrieving the employees who are also sales persons:

 

USE AdventureWorks;
GO
SELECT e.EmployeeID FROM HumanResources.Employee AS e INNER JOIN
Sales.SalesPerson AS s ON e.EmployeeID = s.SalesPersonID

 

The tables or views in the FROM clause can be specified in any order with an inner join or full outer join. However, the order of tables or views specified when using either a left or right outer join is important.
Inner joins return rows only when there is at least one row from both tables that matches the join condition. Inner joins eliminate the rows that do not match with a row from the other table. Outer joins, however, return all rows from at least one of the tables or views mentioned in the FROM clause, as long as those rows meet any WHERE or HAVING search conditions. All rows are retrieved from the left table referenced with a left outer join, and all rows from the right table referenced in a right outer join. All rows from both tables are returned in a full outer join.

4. Can you have a nested transaction?Yes. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and @@TRANCOUNT

5. What is an extended stored procedure? Can you instantiate a COM object by using T-SQL?

An extended stored procedure is a function within a DLL (written in a programming language like C, C++ using Open Data Services (ODS) API) that can be called from T-SQL, just the way we call normal stored procedures using the EXEC statement. See books online to learn how to create extended stored procedures and how to add them to SQL Server. You can instantiate a COM (written in languages like VB, VC++) object from T-SQL by using sp_OACreate stored procedure. Also see books online for sp_OAMethod, sp_OAGetProperty, sp_OASetProperty, sp_OADestroy.

6. What is the system function to get the current user’s user id?

USER_ID(). Also check out other system functions like USER_NAME(), SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(), HOST_NAME().

7. What are triggers? How many triggers you can have on a table? How to invoke a trigger on demand?

Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table. In SQL Server 6.5 you could define only 3 triggers per table, one for INSERT, one for UPDATE and one for DELETE. From SQL Server 7.0 onwards, this restriction is gone, and you could create multiple triggers per each action. But in 7.0 there’s no way to control the order in which the triggers fire. In SQL Server 2000 you could specify which trigger fires first or fires last using sp_settriggerorder. Triggers can’t be invoked on demand. They get triggered only when an associated action (INSERT, UPDATE, DELETE) happens on the table on which they are defined. Triggers are generally used to implement business rules, auditing. Triggers can also be used to extend the referential integrity checks, but wherever possible, use constraints for this purpose, instead of triggers, as constraints are much faster. Till SQL Server 7.0, triggers fire only after the data modification operation happens. So in a way, they are called post triggers. But in SQL Server 2000 you could create pre triggers also. Search SQL Server 2000 books online for INSTEAD OF triggers. Also check out books online for ‘inserted table’, ‘deleted table’ and COLUMNS_UPDATED()

8. There is a trigger defined for INSERT operations on a table, in an OLTP system. The trigger is written to instantiate a COM object and pass the newly inserted rows to it for some custom processing. What do you think of this implementation? Can this be implemented better?

Instantiating COM objects is a time consuming process and since you are doing it from within a trigger, it slows down the data insertion process. Same is the case with sending emails from triggers. This scenario can be better implemented by logging all the necessary data into a separate table, and have a job which periodically checks this table and does the needful.

9. What is a self join? Explain it with an example.

Self join is just like any other join, except that two instances of the same table will be joined in the query. Here is an example: Employees table which contains rows for normal employees as well as managers. So, to find out the managers of all the employees, you need a self join.

CREATE TABLE emp 
(
empid int,
mgrid int,
empname char(10)
)
INSERT emp SELECT 1,2,’Vyas’
INSERT emp SELECT 2,3,’Mohan’
INSERT emp SELECT 3,NULL,’Shobha’
INSERT emp SELECT 4,2,’Shridhar’
INSERT emp SELECT 5,2,’Sourabh’
SELECT t1.empname [Employee], t2.empname [Manager]
FROM emp t1, emp t2
WHERE t1.mgrid = t2.empid
Here’s an advanced query using a LEFT OUTER JOIN that even returns the employees without managers (super bosses)SELECT t1.empname [Employee], COALESCE(t2.empname, ‘No manager’) [Manager]
FROM emp t1
LEFT OUTER JOIN
emp t2
ON
t1.mgrid = t2.empid 

 10. Given an employee table, how would you find out the second highest salary?

CREATE PROC nth 
(
        @table_name sysname,
        @column_name sysname,
        @nth int
)
AS
BEGIN
--Purpose: To find out the nth highest number in a column. 
--Input parameters: Table name, Column name, and the nth position
SET @table_name = RTRIM(@table_name)
SET @column_name = RTRIM(@column_name)
DECLARE @exec_str CHAR(400)
IF (SELECT OBJECT_ID(@table_name,'U')) IS NULL
BEGIN
        RAISERROR('Invalid table name',18,1)
        RETURN -1
END
IF NOT EXISTS(SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS 
WHERE TABLE_NAME = @table_name AND COLUMN_NAME = @column_name)
BEGIN
        RAISERROR('Invalid column name',18,1)
        RETURN -1
END 
IF @nth <= 0
BEGIN
        RAISERROR('nth highest number should be greater than Zero',18,1)
        RETURN -1
END
SET @exec_str = 'SELECT MAX(' + @column_name + ') from ' + @table_name + ' 
WHERE ' + @column_name + ' NOT IN ( SELECT TOP ' + LTRIM(STR(@nth - 1)) + ' ' + @column_name + ' FROM ' + @table_name 
+ ' ORDER BY ' + @column_name + ' DESC )'
EXEC (@exec_str)
END
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