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What is SQL?
Structured Query Language (SQL) is a language that provides an interface to relational database systems.
|Difference between TRUNCATE, DELETE and DROP commands?|
DELETE command is used to remove some or all rows from a table. A WHERE clause can be used to only remove some rows. If no WHERE condition is specified, all rows will be removed. After performing a DELETE operation you need to COMMIT or ROLLBACK the transaction to make the change permanent or to undo it. Note that this operation will cause all DELETE triggers on the table to fire.
TRUNCATE removes all rows from a table. The operation cannot be rolled back and no triggers will be fired. As such, TRUNCATE is faster and doesn't use as much undo space as a DELETE.
DROP command removes a table from the database. All the tables' rows, indexes and privileges will also be removed. No DML triggers will be fired. The operation cannot be rolled back.
DROP and TRUNCATE are DDL commands, whereas DELETE is a DML command. Therefore DELETE operations can be rolled back (undone), while DROP and TRUNCATE operations cannot be rolled back.
DELETE will not free up used space within a table. This means that repeated DELETE commands will severely fragment the table and queries will have to navigate this "free space" in order to retrieve rows.
How does one eliminate duplicate rows from a table?
To remove duplicate rows of data, use the following statement: it will remove duplicate rows from a table and leaving only unique records in the table:
Delete from table_name where rowid not in (select max(rowid) from table group by duplicate_values_field_name);
What the difference between UNION and UNION ALL?
Both UNION and UNION ALL is used to combine results of two SELECT queries.
select * from T1 A 1 1 1 /*union example*/ select * from T1 union select * from T1 A 1 /*union all example*/ select * from T1 union all select * from T1 A 1 1 1 1 1 1
How to Change a SQL prompt name?
SQL> set prompt oracle oracle>
Find out nth highest salary from emp table?
With the help of correlated sub-query, row_number(), dense_rank() and rank() , we can find nth highest salary -
SELECT * FROM Employee order by 2 NAME SALARY Ram 3000 Tom 3000 Vinod 4000 Venky 5000 Manoj 7000 /*correlated sub-query example */ SELECT DISTINCT (a.salary) FROM Employee A WHERE &N = (SELECT COUNT(DISTINCT(b.salary)) FROM Employee B WHERE a.salary <= b.salary); N= 2 -- to identify 2nd highest salary SALARY 5000 /*row_number example */ SELECT DISTINCT (salary) FROM (SELECT e.*, ROW_NUMBER() OVER(ORDER BY salary DESC) rn FROM Employee e) WHERE rn = &N; N= 3 -- to identify 3rd highest salary SALARY 4000 /*correlated sub-query example */ SELECT * FROM (SELECT name, salary, DENSE_RANK() OVER(ORDER BY salary DESC) nth_highest_sal FROM Employee ) WHERE nth_highest_sal = &n; N= 3 -- to identify 3rd highest salary SALARY 4000 /*rank example */ SELECT * FROM (SELECT name, salary, RANK() OVER(ORDER BY salary DESC) nth_highest_sal FROM Employee ) WHERE nth_highest_sal = &n; N= 3 -- to identify 3rd highest salary SALARY 4000
What is the difference between CHAR, VARCHAR and VARCHAR2 data types?
CHAR, VARCHAR and VARCHAR2 are used to store the character string values -
CHAR should be used for storing fixed length character strings. String values will be space/blank padded before stored on disk. CHAR(n) will ALWAYS be n bytes long. If the string length is <n, it will be blank padded upon insert to ensure a length of n.
|What is difference between Co-related sub query and nested sub query?|
Correlated sub query runs once for each row selected by the outer query. It contains a reference to a value from the row selected by the outer query.
Select e1.empname, e1.basicsal, e1.deptno from emp e1 where e1.basicsal = (select max (basicsal) from emp e2 where e2.deptno = e1.deptno)
Nested sub query runs only once for the entire nesting (outer) query. It does not contain any reference to the outer query row.
Select empname, basicsal, deptno from emp where (deptno, basicsal) in (select deptno, max (basicsal) from emp group by deptno)
What are the more common pseudo-columns?
A pseudo-column behaves like a table column but it is not actual column of the table. We can select from pseudo-columns, but we cannot insert, update, or delete their values.
For example - SYSDATE, USER, UID, CURVAL, NEXTVAL, ROWID, ROWNUM, LEVEL
|What is the difference between group by and order by?|
|Group by controls the presentation of the rows, Order by controls the presentation of the columns.|
|What keyword does an SQL SELECT statement use for a string search?|
|The LIKE keyword used for string searches. The % sign is used as a wildcard and _ for single.|
|What are various constraints / Integrity Constraints used in SQL?|
Data integrity allows defining certain data quality requirements that the data in the database needs to meet.
|What is difference between UNIQUE and PRIMARY KEY constraints?|
Difference is that Primary Key have an implicit NOT NULL constraint while Unique Keys do not.
|When do you use WHERE clause and when do you use HAVING clause?|
|WHERE clause applies to the individual rows. HAVING clause is used when you want to specify a condition for a group function and it is written after GROUP BY clause.|
|What are the difference between DDL, DML and DCL commands?|
DDL - Data Definition Language: statements used to define the database structure or schema. Some examples:
|What are sequences? Explain with syntax?|
A field in oracle can be kept as auto incremented by using sequence. it can be used to create a number sequence.
e_seq will cache up to 20 values for performance. Starts from one.
CREATE SEQUENCE e_seq MINVALUE 1 MAXVALUE 99999 START WITH 1 INCREMENT BY 1 CACHE 20;
|General Knowledge of India|
How many coastal state are in India?
A person is to count 4500 currency notes. Let an denote the number of notes he counts in the nth minute. If a1 = a2 = .... = a10 = 150 and a10 , a11 ,... are in an AP with common difference – 2, then the time taken by him to count all notes is ?
The value of product 6 1/2 * 6 1/4 * 6 1/8 * 6 1/16 * -----∞ upto infinite terms?
What is nohup command in unix?
How do display the 10th, 11th and 12th line of a file in Unix?
The mean of 100 students marks was found to be 40. it was found later that a mark 53 was read as 83. the correct mean is?
The geometric mean of three observations 40, 50 and X is 10, then the value of X is ?
Who appoints the members of National human rights commission?
The President of India can resign by writing under his hand addressed to?
Under which article of the constitution of India can the President of India refer a matter to the Supreme Court for its opinion?
|General Computer Knowledge|
The term VPN stands for ?
The transmission control protocol(TCP) is a............ layer protocol ?
What is the difference between Procedure and Function?
Define exception and How will you raise an exception in PL/SQL procedure?
What is the advantage of using packages for storing PL/SQL objects?
|Fill in the blank|
I ________ station before the train arrived.
If I ______ a king, I would help everyone.
Radha___________her daughter for an hour.
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