SQL Interview Questions

1. What are two methods of retrieving SQL?
2. What cursor type do you use to retrieve multiple recordsets?
3. What is the difference between a “where” clause and a “having” clause? - “Where” is a kind of restiriction statement. You use where clause to restrict all the data from DB.Where clause is using before result retrieving. But Having clause is using after retrieving the data.Having clause is a kind of filtering command.
4. What is the basic form of a SQL statement to read data out of a table? The basic form to read data out of table is ‘SELECT * FROM table_name; ‘ An answer: ‘SELECT * FROM table_name WHERE xyz= ‘whatever’;’ cannot be called basic form because of WHERE clause. 
5. What structure can you implement for the database to speed up table reads? - Follow the rules of DB tuning we have to: 1] properly use indexes ( different types of indexes) 2] properly locate different DB objects across different tablespaces, files and so on.3] create a special space (tablespace) to locate some of the data with special datatype ( for example CLOB, LOB and …)
6. What are the tradeoffs with having indexes? – 1. Faster selects, slower updates. 2. Extra storage space to store indexes. Updates are slower because in addition to updating the table you have to update the index.
7. What is a “join”? - ‘join’ used to connect two or more tables logically with or without common field.
8. What is “normalization”? “Denormalization”? Why do you sometimes want to denormalize? - Normalizing data means eliminating redundant information from a table and organizing the data so that future changes to the table are easier. Denormalization means allowing redundancy in a table. The main benefit of denormalization is improved performance with simplified data retrieval and manipulation. This is done by reduction in the number of joins needed for data processing.
9. What is a “constraint”? - A constraint allows you to apply simple referential integrity checks to a table. There are four primary types of constraints that are currently supported by SQL Server: PRIMARY/UNIQUE – enforces uniqueness of a particular table column. DEFAULT – specifies a default value for a column in case an insert operation does not provide one. FOREIGN KEY – validates that every value in a column exists in a column of another table. CHECK – checks that every value stored in a column is in some specified list. Each type of constraint performs a specific type of action. Default is not a constraint. NOT NULL is one more constraint which does not allow values in the specific column to be null. And also it the only constraint which is not a table level constraint.
10. What types of index data structures can you have? - An index helps to faster search values in tables. The three most commonly used index-types are: – B-Tree: builds a tree of possible values with a list of row IDs that have the leaf value. Needs a lot of space and is the default index type for most databases. – Bitmap: string of bits for each possible value of the column. Each bit string has one bit for each row. Needs only few space and is very fast.(however, domain of value cannot be large, e.g. SEX(m,f); degree(BS,MS,PHD) – Hash: A hashing algorithm is used to assign a set of characters to represent a text string such as a composite of keys or partial keys, and compresses the underlying data. Takes longer to build and is supported by relatively few databases.
11. What is a “primary key”? - A PRIMARY INDEX or PRIMARY KEY is something which comes mainly from 
    database theory. From its behavior is almost the same as an UNIQUE INDEX, i.e. there may only be one of each value in this column. If you call such an INDEX PRIMARY instead of UNIQUE, you say something about 
    your table design, which I am not able to explain in few words. Primary Key is a type of a constraint enforcing uniqueness and data integrity for each row of a table. All columns participating in a primary key constraint must possess the NOT NULL property.
12. What is a “functional dependency”? How does it relate to database table design? – Functional dependency relates to how one object depends upon the other in the database. for example, procedure/function sp2 may be called by procedure sp1. Then we say that sp1 has functional dependency on sp2.
13. What is a “trigger”? - Triggers are stored procedures created in order to enforce integrity rules in a database. A trigger is executed every time a data-modification operation occurs (i.e., insert, update or delete). Triggers are executed automatically on occurance of one of the data-modification operations. A trigger is a database object directly associated with a particular table. It fires whenever a specific statement/type of statement is issued against that table. The types of statements are insert,update,delete and query statements. Basically, trigger is a set of SQL statements A trigger is a solution to the restrictions of a constraint. For instance: 1.A database column cannot carry PSEUDO columns as criteria where a trigger can. 2. A database constraint cannot refer old and new values for a row where a trigger can.
14. Why can a “group by” or “order by” clause be expensive to process? – Processing of “group by” or “order by” clause often requires creation of Temporary tables to process the results of the query. Which depending of the result set can be very expensive.
15. What is “index covering” of a query? – Index covering means that “Data can be found only using indexes, without touching the tables”
16. What types of join algorithms can you have?
17. What is a SQL view? – An output of a query can be stored as a view. View acts like small table which meets our criterion. View is a precomplied SQL query which is used to select data from one or more tables. A view is like a table but it doesn’t physically take any space. View is a good way to present data in a particular format if you use that query quite often. View can also be used to restrict users from accessing the tables directly.

 

1. Which of the following has the highest order of precedence?
   • Functions and Parenthesis
   • Multiplication, Division and Exponents
   • Addition and Subtraction
   • Logical Operations
2. When designing a database table, how do you avoid missing column values for non-primary key columns?
   • Use UNIQUE constraints
   • Use PRIMARY KEY constraints
   • Use DEFAULT and NOT NULL constraints
   • Use FOREIGN KEY constraints
   • Use SET constraints
3. Which of the following is the syntax for creating an Index?
   • CREATE [UNIQUE] INDEX index_name OF tbl_name (index_columns)
   • CREATE [UNIQUE] INDEX OF tbl_name (index_columns)
   • CREATE [UNIQUE] INDEX ON tbl_name (index_columns)
   • CREATE [UNIQUE] INDEX index_name ON tbl_name (index_columns)
4. Which of the following is not a valid character datatype in SQL Server?
   • BLOB
   • CHAR
   • VARCHAR
   • TEXT
   • VARTEXT
5. Which of the following statements about SQL Server comments is false?
   • /* … */ are used for multiline comments
   • // is used for single line comments
   • – is used for single line comments
   • Nested comments are allowed i.e. /* comment 1 /* comment 2 */ comment 1*/
   • ‘ is used for single line comments
6. Consider the following transaction code:
   
   Begin Transaction
Update names_table set employee_name = "Ramesh" where employee_name = "Mahesh"
Save Transaction SAVE_POINT
Update salaries set salary=salary + 900 where employee_job = "Engineer"
Rollback transaction
Commit transaction
   What will be the result produced by this transaction?   
   • “Ramesh” will be updated to “Mahesh”, but salaries of engineers will not be

updated

1.  
   • Neither “Ramesh” will be updated to “Mahesh”, nor the salary of engineers will be updated.
   • “Ramesh” will be updated to “Mahesh” and salary of engineers will also be

updated.

1. Which of the following constraints can be used to enforce the uniqueness of rows in a table?
   • DEFAULT and NOT NULL constraints
   • FOREIGN KEY constraints
   • PRIMARY KEY and UNIQUE constraints
   • IDENTITY columns
   • CHECK constraints
2. Which of the following are not date parts?
   • quarter
   • dayofweek
   • dayofyear
   • weekday
3. The IF UPDATE (column_name) parameter in a trigger definition will return
   TRUE in case of an INSERT statement being executed on the triggered table:   
   • Yes
   • No
   • It returns TRUE only if an UPDATE query is executed
   • Both b and c
4. Which one of the following must be specified in every DELETE statement?
   • Table Name
   • Database name
   • LIMIT clause
   • WHERE clause
   • Column Names
5. Which one of the following correctly selects rows from the table myTable that have null in column column1?
   • SELECT * FROM myTable WHERE column1 is null
   • SELECT * FROM myTable WHERE column1 = null
   • SELECT * FROM myTable WHERE column1 EQUALS null
   • SELECT * FROM myTable WHERE column1 NOT null
   • SELECT * FROM myTable WHERE column1 CONTAINS null
6. Is this statement true or false:
   A cursor is a pointer that identifies a specific working row within a set   
   • True
   • False
7. Which of the following commands is used to change the structure of table?
   • CHANGE TABLE
   • MODIFY TABLE
   • ALTER TABLE
   • UPDATE TABLE
8. Consider the following statements and pick the correct answer:
   
   1. ceiling() - returns the smallest integer greater than or equal to the specified value
2. floor() - returns the largest integer less than or equal to the specified value   
   • 1 is true and 2 is false
   • 1 is false and 2 is true
   • Both 1 and 2 are true
   • Both 1 and 2 are false
9. What is the correct SQL syntax for returning all the columns from a table named “Persons” sorted REVERSE alphabetically by “FirstName”?
   • SELECT * FROM Persons WHERE FirstName ORDER BY FirstName DESC
   • SELECT * FROM Persons SORT REVERSE ‘FirstName’

c . SELECT * FROM Persons ORDER BY ‘FirstName’

1.  
   • SELECT * FROM Persons ORDER BY FirstName DESC

SELECT * FROM Persons ORDER BY DESC FirstName

1. What is the maximum value that can be stored for a datetime field?
   • Dec 31, 9999
   • Jun 6, 2079
   • Jan 1, 2753
   • Jan 1, 2100
2. Consider the following queries:
   1. select * from employee where department LIKE "[^F-M]%”;
2. select * from employee where department = “[^F-M]%”;
   Select the correct option:   
   • Query 2 will return an error
   • Both the queries will return the same set of records
   • Query 2 is perfectly correct
   • Query 2 would return one record less than Query 1
3. How can you view the structure of a table named “myTable” in SQL Server?
   • desc myTable
   • desc table myTable
   • sp_columns myTable
   • None of the above
   • Using either option a or c
4. What does referential integrity (also called relational integrity) prevent?
   • Loss of data from employee sabotage
   • Loss of data from any one corrupted table
   • Recursive joins
   • One-to-many or many-to-many relationships between columns in a table
   • Data redundancy
5. Which of the following is not a global variable?
   • @@colcount
   • @@error
   • @@rowcount
   • @@version
   • All are valid global variables
6. Consider the following two tables:1. customers( customer_id, customer_name)
   2. branch ( branch_id, branch_name )
   What will be the output if the following query is executed:
   Select * branch_name from customers,branch   
   • It will return the fields customer_id, customer_name, branch_name
   • It will return the fields customer_id, customer_name, branch_id, branch_name
   • It will return the fields customer_id, customer_name, branch_id, branch_name, branch_name
   • It will return an empty set since the two tables do not have any common field name
   • It will return an error since * is used alone for one table only

    

    

    

7. Which of the following is not a control statement?
   • if…else
   • if exists
   • do…while
   • while
   • begin…end
8. Which of the following is not a valid Numeric datatypes in SQL Server?
   • INT
   • SMALLINT
   • TINYINT
   • BIGINT
   • MONEY
9. Which of the following datatypes is not supported by SQL-Server?
   • Character
   • Binary
   • Logical
   • Date
   • Numeric
   • All are supported
10. What will the output be if you try to perform arithmetic on NULL values?
    • 0
    • NULL
    • It will generate an error message
    • Can’t be determined
11. Which of the following options is not correct about the DATEDIFF() function?
    • It returns the difference between parts of two specified dates
    • It takes three arguments
    • It returns a signed integer value equal to second date part minus first date part
    • It returns a signed integer value equal to first date part minus second date part
12. Sample Code
    
    CREATE TABLE table1(
column1 varchar(50),
column2 varchar(50),
column3 varchar(50),
column4 varchar(50));
    Which one of the following is the correct syntax for adding the column named “column2a” after column2 to the table shown above?   
    • ALTER TABLE table1 ADD column2a varchar(50) AFTER column2;
    • MODIFY TABLE table1 ADD column2a AFTER column2;
    • INSERT INTO table1 column2a AS varchar(50) AFTER column2;
    • ALTER TABLE table1 INSERT column2a varchar(50) AFTER column2;
    • CHANGE TABLE table1 INSERT column2a BEFORE column3;
13. State which of the following are true
    • Views are a logical way of looking at the logical data located in the tables
    • Views are a logical way of looking at the physical data located in the tables
    • Tables are physical constructs used for storage and manipulation of data in databases
    • Tables are logical constructs used for storage and manipulation of data in databases
14. Which of the following is not a valid binary datatype in SQL Server?
    • BINARY
    • VARBINARY
    • BIT
    • IMAGE
    • TESTAMP
15. Which of the following is false with regards to sp_help?
    • When a procedure name is passed to sp_help, it shows the parameters
    • When a table name is passed to sp_help, it shows the structure of the table
    • When no parameter is passed, it provides a list of all objects and user-defined datatypes in a database
    • All of the above are true
    • Which of the following are false for batches (batch commands)?
      • Statements in a batch are parsed, compiled and executed as a group
      • None of the statements in the batch is executed if there are any syntax errors in the batch
      • None of the statements in the batch is executed if there are any parsing errors in the batch
      • None of the statements in the batch is executed if there are any fatal errors in the batch
    • Select the correct option:
      • Optimistic locking is a locking scheme handled by the server, whereas pessimistic locking is handled by the application developer
      • Pessimistic locking is a locking scheme handled by the server, whereas optimistic locking is handled by the application developer