Structured Query Language Interview Questions and answers

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  What is SQL and where does it come from?
Structured Query Language (SQL) is a language that provides an interface to relational database systems. SQL was developed by IBM in the 1970s for use in System R, and is a de facto standard, as well as an ISO and ANSI standard. SQL is often pronounced SEQUEL.
In common usage SQL also encompasses DML (Data Manipulation Language), for INSERTs, UPDATEs, DELETEs and DDL (Data Definition Language), used for creating and modifying tables and other database structures.
The development of SQL is governed by standards. A major revision to the SQL standard was completed in 1992, called SQL2. SQL3 support object extensions and will be (partially?) implemented in Oracle8.


What are the difference between DDL, DML and DCL commands?
DDL is Data Definition Language statements. Some examples:
 CREATE - to create objects in the database
 ALTER - alters the structure of the database
 DROP - delete objects from the database
 TRUNCATE - remove all records from a table, including all spaces allocated for the records are removed
 COMMENT - add comments to the data dictionary
 GRANT - gives user's access privileges to database
 REVOKE - withdraw access privileges given with the GRANT command
DML is Data Manipulation Language statements. Some examples:
 SELECT - retrieve data from the a database
 INSERT - insert data into a table
 UPDATE - updates existing data within a table
 DELETE - deletes all records from a table, the space for the records remain
 CALL - call a PL/SQL or Java subprogram
 EXPLAIN PLAN - explain access path to data
 LOCK TABLE - control concurrency
DCL is Data Control Language statements. Some examples:
 COMMIT - save work done
 SAVEPOINT - identify a point in a transaction to which you can later roll back
 ROLLBACK - restore database to original since the last COMMIT
 SET TRANSACTION - Change transaction options like what rollback segment to use
How can I eliminate duplicates values in a table?
Choose one of the following queries to identify or remove duplicate rows from a table leaving one record:
Method 1:
SQL> DELETE FROM table_name A WHERE ROWID > (
2 SELECT min(rowid) FROM table_name B
3 WHERE A.key_values = B.key_values);
Method 2:
SQL> create table table_name2 as select distinct * from table_name1;
SQL> drop table_name1;
SQL> rename table_name2 to table_name1;
Method 3:
SQL> Delete from my_table where rowid not in(
SQL> select max(rowid) from my_table
SQL> group by my_column_name );
Method 4:
SQL> delete from my_table t1
SQL> where exists (select 'x' from my_table t2
SQL> where t2.key_value1 = t1.key_value1
SQL> and t2.key_value2 = t1.key_value2
SQL> and t2.rowid > t1.rowid);
Note: If you create an index on the joined fields in the inner loop, you, for all intents purposes, eliminate N^2 operations (no need to loop through the entire table on each pass by a record).
How can I generate primary key values for my table?
Create your table with a NOT NULL column (say SEQNO). This column can now be populated with unique values:
SQL> UPDATE table_name SET seqno = ROWNUM;
or use a sequences generator:
SQL> CREATE SEQUENCE sequence_name START WITH 1 INCREMENT BY 1;
SQL> UPDATE table_name SET seqno = sequence_name.NEXTVAL;

Finally, create a unique index on this column.
How can I get the time difference between two date columns
Look at this example query:
select floor(((date1-date2)*24*60*60)/3600)
|| ' HOURS ' ||
floor((((date1-date2)*24*60*60) -
floor(((date1-date2)*24*60*60)/3600)*3600)/60)
|| ' MINUTES ' ||
round((((date1-date2)*24*60*60) -
floor(((date1-date2)*24*60*60)/3600)*3600 -
(floor((((date1-date2)*24*60*60) -
floor(((date1-date2)*24*60*60)/3600)*3600)/60)*60)))
|| ' SECS ' time_difference
from ...
How does one count different data values in a column?

select dept, sum( decode(sex,'M',1,0)) MALE,
sum( decode(sex,'F',1,0)) FEMALE,
count(decode(sex,'M',1,'F',1)) TOTAL
from my_emp_table
group by dept;
How does one count/sum RANGES of data values in a column?
A value x will be between values y and z if GREATEST(x, y) = LEAST(x, z). Look at this example:
select f2,
sum(decode(greatest(f1,59), least(f1,100), 1, 0)) "Range 60-100",
sum(decode(greatest(f1,30), least(f1, 59), 1, 0)) "Range 30-59",
sum(decode(greatest(f1, 0), least(f1, 29), 1, 0)) "Range 00-29"
from my_table
group by f2;
For equal size ranges it might be easier to calculate it with DECODE(TRUNC(value/range), 0, rate_0, 1, rate_1, ...). Eg.
select ename "Name", sal "Salary",
decode( trunc(f2/1000, 0), 0, 0.0,
1, 0.1,
2, 0.2,
3, 0.31) "Tax rate"
from my_table;
Can one retrieve only the Nth row from a table?
provided this solution to select the Nth row form a table:
SELECT f1 FROM t1
WHERE rowid = (
SELECT rowid FROM t1
WHERE rownum <= 10
MINUS
SELECT rowid FROM t1
WHERE rownum < 10);
Alternatively...
SELECT * FROM emp WHERE rownum=1 AND rowid NOT IN
(SELECT rowid FROM emp WHERE rownum < 10);
Please note, there is no explicit row order in a relational database. However, this query is quite fun and may even help in the odd situation.
Can one retrieve only rows X to Y from a table?
To display rows 5 to 7, construct a query like this:
SELECT *
FROM tableX
WHERE rowid in (
SELECT rowid FROM tableX
WHERE rownum <= 7
MINUS
SELECT rowid FROM tableX
WHERE rownum < 5);
Please note, there is no explicit row order in a relational database. However, this query is quite fun and may even help in the odd situation.
How does one select EVERY Nth row from a table?
One can easily select all even, odd, or Nth rows from a table using SQL queries like this:
Method 1: Using a subquery
SELECT *
FROM emp
WHERE (ROWID,0) IN (SELECT ROWID, MOD(ROWNUM,4)
FROM emp);
Method 2: Use dynamic views (available from Oracle7.2):
SELECT *
FROM ( SELECT rownum rn, empno, ename
FROM emp
) temp
WHERE MOD(temp.ROWNUM,4) = 0;
Please note, there is no explicit row order in a relational database. However, these queries are quite fun and may even help in the odd situation.
How does one select the TOP N rows from a table?
Form Oracle8i one can have an inner-query with an ORDER BY clause. Look at this example:
SELECT *
FROM (SELECT * FROM my_table ORDER BY col_name_1 DESC)
WHERE ROWNUM < 10;
Use this workaround with prior releases:
SELECT *
FROM my_table a
WHERE 10 >= (SELECT COUNT(DISTINCT maxcol)
FROM my_table b
WHERE b.maxcol >= a.maxcol)
ORDER BY maxcol DESC;
How does one code a tree-structured query?
Tree-structured queries are definitely non-relational (enough to kill Codd and make him roll in his grave). Also, this feature is not often found in other database offerings.
The SCOTT/TIGER database schema contains a table EMP with a self-referencing relation (EMPNO and MGR columns). This table is perfect for tesing and demonstrating tree-structured queries as the MGR column contains the employee number of the "current" employee's boss.
The LEVEL pseudo-column is an indication of how deep in the tree one is. Oracle can handle queries with a depth of up to 255 levels. Look at this example:
select LEVEL, EMPNO, ENAME, MGR
from EMP
connect by prior EMPNO = MGR
start with MGR is NULL;
One can produce an indented report by using the level number to substring or lpad() a series of spaces, and concatenate that to the string. Look at this example:
select lpad(' ', LEVEL * 2) || ENAME ........
One uses the "start with" clause to specify the start of the tree. More than one record can match the starting condition. One disadvantage of having a "connect by prior" clause is that you cannot perform a join to other tables. The "connect by prior" clause is rarely implemented in the other database offerings. Trying to do this programmatically is difficult as one has to do the top level query first, then, for each of the records open a cursor to look for child nodes.
One way of working around this is to use PL/SQL, open the driving cursor with the "connect by prior" statement, and the select matching records from other tables on a row-by-row basis, inserting the results into a temporary table for later retrieval.
How does one code a matrix report in SQL?
Look at this example query with sample output:
SELECT *
FROM (SELECT job,
sum(decode(deptno,10,sal)) DEPT10,
sum(decode(deptno,20,sal)) DEPT20,
sum(decode(deptno,30,sal)) DEPT30,
sum(decode(deptno,40,sal)) DEPT40
FROM scott.emp
GROUP BY job)
ORDER BY 1;

JOB DEPT10 DEPT20 DEPT30 DEPT40
--------- ---------- ---------- ---------- ----------
ANALYST 6000
CLERK 1300 1900 950
MANAGER 2450 2975 2850
PRESIDENT 5000
SALESMAN 5600
How does one implement IF-THEN-ELSE in a select statement?
The Oracle decode function acts like a procedural statement inside an SQL statement to return different values or columns based on the values of other columns in the select statement.
Some examples:
select decode(sex, 'M', 'Male',
'F', 'Female',
'Unknown')
from employees;

select a, b, decode( abs(a-b), a-b, 'a > b',
0, 'a = b',
'a < b')
from tableX;

select decode( GREATEST(A,B), A, 'A is greater than B', 'B is greater than A')...
Note: The decode function is not ANSI SQL and is rarely implemented in other RDBMS offerings. It is one of the good things about Oracle, but use it sparingly if portability is required.
From Oracle 8i one can also use CASE statements in SQL. Look at this example:
SELECT ename, CASE WHEN sal>1000 THEN 'Over paid' ELSE 'Under paid' END
FROM emp;
How can one dump/ examine the exact content of a database column?

SELECT DUMP(col1)
FROM tab1
WHERE cond1 = val1;

DUMP(COL1)
-------------------------------------
Typ=96 Len=4: 65,66,67,32
For this example the type is 96, indicating CHAR, and the last byte in the column is 32, which is the ASCII code for a space. This tells us that this column is blank-padded.



Can one drop a column from a table?
From Oracle8i one can DROP a column from a table. Look at this sample script, demonstrating the ALTER TABLE table_name DROP COLUMN column_name; command.
With previous releases one can use Joseph S. Testa's DROP COLUMN package that can be downloaded from http://www.oracle-dba.com/ora_scr.htm.
Other workarounds:
1. SQL> update t1 set column_to_drop = NULL;
SQL> rename t1 to t1_base;
SQL> create view t1 as select from t1_base;

2. SQL> create table t2 as select from t1;
SQL> drop table t1;
SQL> rename t2 to t1;
Can one rename a column in a table?
No, this is listed as Enhancement Request 163519. Some workarounds:
1. -- Use a view with correct column names...
rename t1 to t1_base;
create view t1 as select * from t1_base;

2. -- Recreate the table with correct column names...
create table t2 as select * from t1;
drop table t1;
rename t2 to t1;

3. -- Add a column with a new name and drop an old column...
alter table t1 add ( newcolame datatype );
update t1 set newcolname=oldcolname;
alter table t1 drop column oldcolname;
How can I change my Oracle password?
Issue the following SQL command: ALTER USER IDENTIFIED BY
/
From Oracle8 you can just type "password" from SQL*Plus, or if you need to change another user's password, type "password user_name".
How does one find the next value of a sequence?
Perform an "ALTER SEQUENCE ... NOCACHE" to unload the unused cached sequence numbers from the Oracle library cache. This way, no cached numbers will be lost. If you then select from the USER_SEQUENCES dictionary view, you will see the correct high water mark value that would be returned for the next NEXTVALL call. Afterwards, perform an "ALTER SEQUENCE ... CACHE" to restore caching.
You can use the above technique to prevent sequence number loss before a SHUTDOWN ABORT, or any other operation that would cause gaps in sequence values.
Workaround for snapshots on tables with LONG columns
You can use the SQL*Plus COPY command instead of snapshots if you need to copy LONG and LONG RAW variables from one location to another. Eg:
COPY TO SCOTT/TIGER@REMOTE -
CREATE IMAGE_TABLE USING -
SELECT IMAGE_NO, IMAGE -
FROM IMAGES;
Note: If you run Oracle8, convert your LONGs to LOBs, as it can be replicated.