SQL Interview Questions and Answers
1) What are the components of the physical database structure of Oracle database?
Ans: Oracle database is comprised of three types of files. One or more datafiles, two are more redo log files, and one or more control files.
2) What are the components of the logical database structure of Oracle database?
Ans: There are tablespaces and database’s schema objects.
3) What is a tablespace?
Ans: A database is divided into Logical Storage Unit called tablespaces. A tablespace is used to grouped related logical structures together.
4) What is SYSTEM tablespace and when is it created?
Ans: Every Oracle database contains a tablespace named SYSTEM, which is automatically created when the database is created. The SYSTEM tablespace always contains the data dictionary tables for the entire database.
5) Explain the relationship between database, tablespace and data file.
Ans: Each database logically divided into one or more tablespaces one or more data files are explicitly created for each tablespace.
6) What is schema?
Ans: A schema is a collection of database objects of a user.
7) What are Schema Objects?
Ans: Schema objects are the logical structures that directly refer to the database’s data. Schema objects include tables, views, sequences, synonyms, indexes, clusters, database triggers, procedures, functions packages and database links.
8) Can objects of the same schema reside in different table spaces?
9) Can a tablespace hold objects from different schemes?
10) What is Oracle table?
Ans: A table is the basic unit of data storage in an Oracle database. The tables of a database hold all of the user accessible data. Table data is stored in rows and columns.
11) What is an Oracle view?
Ans: A view is a virtual table. Every view has a query attached to it. (The query is a SELECT statement that identifies the columns and rows of the table(s) the view uses.)
12) Do a view contain data?
Ans: Views do not contain or store data.
13) Can a view based on another view?
14) What are the advantages of views?
Ans: Provide an additional level of table security, by restricting access to a predetermined set of rows and columns of a table.
– Hide data complexity.
– Simplify commands for the user.
– Present the data in a different perspective from that of the base table.
– Store complex queries.
15) What is an Oracle sequence?
Ans: A sequence generates a serial list of unique numbers for numerical columns of a database’s tables.
16) What is a synonym?
Ans: A synonym is an alias for a table, view, sequence or program unit.
17) What are the types of synonyms?
Ans: There are two types of synonyms private and public.
18) What is a private synonym?
Ans: Only its owner can access a private synonym.
19) What is a public synonym?
Ans: Any database user can access a public synonym.
20) What are synonyms used for?
Ans: Mask the real name and owner of an object.
– Provide public access to an object
– Provide location transparency for tables, views or program units of a remote database.
– Simplify the SQL statements for database users.
21) What is an Oracle index?
Ans: An index is an optional structure associated with a table to have direct access to rows, which can be created to increase the performance of data retrieval. Index can be created on one or more columns of a table.
22) How are the index updates?
Ans: Indexes are automatically maintained and used by Oracle. Changes to table data are automatically incorporated into all relevant indexes.
23) What are clusters?
Ans: Clusters are groups of one or more tables physically stores together to share common columns and are often used together.
24) What is cluster key?
Ans: The related columns of the tables in a cluster are called the cluster key.
25) What is index cluster?
Ans: A cluster with an index on the cluster key.
26) What is hash cluster?
Ans: A row is stored in a hash cluster based on the result of applying a hash function to the row’s cluster key value. All rows with the same hash key value are stores together on disk.
27) When can hash cluster used?
Ans: Hash clusters are better choice when a table is often queried with equality queries. For such queries the specified cluster key value is hashed. The resulting hash key value points directly to the area on disk that stores the specified rows.
28) What is database link?
Ans: A database link is a named object that describes a “path” from one database to another.
29) What are the types of database links?
Ans: Private database link, public database link & network database link.
30) What is private database link?
Ans: Private database link is created on behalf of a specific user. A private database link can be used only when the owner of the link specifies a global object name in a SQL statement or in the definition of the owner’s views or procedures.
31) What is public database link?
Ans: Public database link is created for the special user group PUBLIC. A public database link can be used when any user in the associated database specifies a global object name in a SQL statement or object definition.
32) What is network database link?
Ans: Network database link is created and managed by a network domain service. A network database link can be used when any user of any database in the network specifies a global object name in a SQL statement or object definition.
33) What is data block?
Ans: Oracle database’s data is stored in data blocks. One data block corresponds to a specific number of bytes of physical database space on disk.
34) How to define data block size?
Ans: A data block size is specified for each Oracle database when the database is created. A database users and allocated free database space in Oracle data blocks. Block size is specified in init.ora file and cannot be changed latter.
35) What is row chaining?
Ans: In circumstances, all of the data for a row in a table may not be able to fit in the same data block. When this occurs, the data for the row is stored in a chain of data block (one or more) reserved for that segment.
36) What is an extent?
Ans: An extent is a specific number of contiguous data blocks, obtained in a single allocation and used to store a specific type of information.
37) What is a segment?
Ans: A segment is a set of extents allocated for a certain logical structure.
38) What are the different types of segments?
Ans: Data segment, index segment, rollback segment and temporary segment.
39) What is a data segment?
Ans: Each non-clustered table has a data segment. All of the table’s data is stored in the extents of its data segment. Each cluster has a data segment. The data of every table in the cluster is stored in the cluster’s data segment.
40) What is an index segment?
Ans: Each index has an index segment that stores all of its data.
41) What is rollback segment?
Ans: A database contains one or more rollback segments to temporarily store “undo” information.
42) What are the uses of rollback segment?
Ans: To generate read-consistent database information during database recovery and to rollback uncommitted transactions by the users.
43) What is a temporary segment?
Ans: Temporary segments are created by Oracle when a SQL statement needs a temporary work area to complete execution. When the statement finishes execution, the temporary segment extents are released to the system for future use.
44) What is a datafile?
Ans: Every Oracle database has one or more physical data files. A database’s data files contain all the database data. The data of logical database structures such as tables and indexes is physically stored in the data files allocated for a database.
45) What are the characteristics of data files?
Ans: A data file can be associated with only one database. Once created a data file can’t change size. One or more data files form a logical unit of database storage called a tablespace.
46) What is a redo log?
Ans: The set of redo log files for a database is collectively known as the database redo log.
47) What is the function of redo log?
Ans: The primary function of the redo log is to record all changes made to data.
48) What is the use of redo log information?
Ans: The information in a redo log file is used only to recover the database from a system or media failure prevents database data from being written to a database’s data files.
49) What does a control file contains?
– Database name
– Names and locations of a database’s files and redolog files.
– Time stamp of database creation.
50) What is the use of control file?
Ans: When an instance of an Oracle database is started, its control file is used to identify the database and redo log files that must be opened for database operation to proceed. It is also used in database recovery.
Data Base Administration
51) What is a database instance? Explain.
Ans: A database instance (Server) is a set of memory structure and background processes that access a set of database files. The processes can be shared by all of the users.
The memory structure that is used to store the most queried data from database. This helps up to improve database performance by decreasing the amount of I/O performed against data file.
52) What is Parallel Server?
Ans: Multiple instances accessing the same database (only in multi-CPU environments)
53) What is a schema?
Ans: The set of objects owned by user account is called the schema.
54) What is an index? How it is implemented in Oracle database?
Ans: An index is a database structure used by the server to have direct access of a row in a table. An index is automatically created when a unique of primary key constraint clause is specified in create table command
55) What are clusters?
Ans: Group of tables physically stored together because they share common columns and are often used together is called cluster.
56) What is a cluster key?
Ans: The related columns of the tables are called the cluster key. The cluster key is indexed using a cluster index and its value is stored only once for multiple tables in the cluster.
57) What is the basic element of base configuration of an Oracle database?
Ans: It consists of
one or more data files.
one or more control files.
two or more redo log files.
The Database contains
one or more rollback segments
one or more tablespaces
Data dictionary tables
User objects (table,indexes,views etc.,)
The server that access the database consists of
SGA (Database buffer, Dictionary Cache Buffers, Redo log buffers, Shared SQL pool)
SMON (System MONito)
PMON (Process MONitor)
LGWR (LoG Write)
DBWR (Data Base Write)
CKPT (Check Point)
User Process with associated PGS
58) What is a deadlock? Explain.
Ans: Two processes waiting to update the rows of a table, which are locked by other processes then deadlock arises.
In a database environment this will often happen because of not issuing the proper row lock commands. Poor design of front-end application may cause this situation and the performance of server will reduce drastically.
These locks will be released automatically when a commit/rollback operation performed or any one of this processes being killed externally.
59) What is SGA?
Ans: The System Global Area in an Oracle database is the area in memory to facilitate the transfer of information between users. It holds the most recently requested structural information between users. It holds the most recently requested structural information about the database. The structure is database buffers, dictionary cache, redo log buffer and shared pool area.
60) What is a shared pool?
Ans: The data dictionary cache is stored in an area in SGA called the shared pool. This will allow sharing of parsed SQL statements among concurrent users.
61) What is mean by Program Global Area (PGA)?
Ans: It is area in memory that is used by a single Oracle user process.
62) What is a data segment?
Ans: Data segment are the physical areas within a database block in which the data associated with tables and clusters are stored.
63) What are the factors causing the reparsing of SQL statements in SGA?
Ans: Due to insufficient shared pool size.
Monitor the ratio of the reloads takes place while executing SQL statements. If the ratio is greater than 1 then increase the SHARED_POOL_SIZE.
Database Logical & Physical Architecture
64) What is Database Buffers?
Ans: Database buffers are cache in the SGA used to hold the data blocks that are read from the data segments in the database such as tables, indexes and clusters DB_BLOCK_BUFFERS parameter in INIT.ORA decides the size.
65) What is dictionary cache?
Ans: Dictionary cache is information about the database objects stored in a data dictionary table.
66) What is meant by recursive hints?
Ans: Number of times processes repeatedly query the dictionary table is called recursive hints. It is due to the data dictionary cache is too small. By increasing the SHARED_POOL_SIZE parameter we can optimize the size of data dictionary cache.
67) What is redo log buffer?
Ans: Changes made to the records are written to the on-line redo log files. So that they can be used in roll forward operations during database recoveries. Before writing them into the redo log files, they will first brought to redo log buffers in SGA and LGWR will write into files frequently. LOG_BUFFER parameter will decide the size.
68) How will you swap objects into a different table space for an existing database?
– Export the user
– Perform import using the command imp system/manager file=export.dmp indexfile=newrite.sql. This will create all definitions into newfile.sql.
– Drop necessary objects.
– Run the script newfile.sql after altering the tablespaces.
– Import from the backup for the necessary objects.
69) List the Optional Flexible Architecture (OFA) of Oracle database? How can we organize the tablespaces in Oracle database to have maximum performance?
Ans: SYSTEM – Data dictionary tables.
DATA – Standard operational tables.
DATA2- Static tables used for standard operations
INDEXES – Indexes for Standard operational tables.
INDEXES1 – Indexes of static tables used for standard operations.
TOOLS – Tools table.
TOOLS1 – Indexes for tools table.
RBS – Standard Operations Rollback Segments,
RBS1,RBS2 – Additional/Special Rollback segments.
TEMP – Temporary purpose tablespace
TEMP_USER – Temporary tablespace for users.
USERS – User tablespace.
70) How will you force database to use particular rollback segment?
Ans: SET TRANSACTION USE ROLLBACK SEGMENT rbs_name.
71) What is meant by free extent?
Ans: A free extent is a collection of continuous free blocks in tablespace. When a segment is dropped its extents are reallocated and are marked as free.
72) Which parameter in Storage clause will reduce number of rows per block?
Ans: PCTFREE parameter
Row size also reduces no of rows per block.
73) What is the significance of having storage clause?
Ans: We can plan the storage for a table as how much initial extents are required, how much can be extended next, how much % should leave free for managing row updating, etc.,
74) How does Space allocation table place within a block?
Ans: Each block contains entries as follows
Fixed block header
Variable block header
Row Header, row date (multiple rows may exists)
PCTEREE (% of free space for row updating in future)
75) What is the role of PCTFREE parameter is storage clause?
Ans: This is used to reserve certain amount of space in a block for expansion of rows.
76) What is the OPTIMAL parameter?
Ans: It is used to set the optimal length of a rollback segment.
77) What is the functionality of SYSTEM table space?
Ans: To manage the database level transactions such as modifications of the data dictionary table that record information about the free space usage.
78) How will you create multiple rollback segments in a database?
– Create a database, which implicitly creates a SYSTEM rollback segment in a SYSTEM tablespace.
– Create a second rollback segment name R0 in the SYSTEM tablespace.
– Make new rollback segment available (after shutdown, modify init.ora file and start database)
– Create other tablespaces (RBS) for rollback segments.
– Deactivate rollback segment R0 and activate the newly created rollback segments.
79) How the space utilization takes place within rollback segments?
Ans: It will try to fit the transaction in a cyclic fashion to all existing extents. Once it found an extent is in use then it forced to acquire a new extent (number of extents is based on the optimal size)
80) Why query fails sometimes?
Ans: Rollback segment dynamically extent to handle larger transactions entry loads.
A single transaction may wipeout all available free space in the rollback segment tablespace. This prevents other user using rollback segments.
81) How will you monitor the space allocation?
Ans: By querying DBA_SEGMENT table/view
82) How will you monitor rollback segment status?
Ans: Querying the DBA_ROLLBACK_SEGS view
IN USE – Rollback Segment is on-line.
AVAILABLE – Rollback Segment available but not on-line.
OFF-LINE – Rollback Segment off-line
INVALID – Rollback Segment Dropped.
NEEDS RECOVERY – Contains data but need recovery or corrupted.
PARTLY AVAILABLE – Contains data from an unresolved transaction involving a
83) List the sequence of events when a large transaction that exceeds beyond its optimal value when an entry wraps and causes the rollback segment to expand into another extend.
Ans: Transaction Begins.
An entry is made in the RES header for new transactions entry
Transaction acquires blocks in an extent of RBS
The entry attempts to wrap into second extent. None is available, so that the RBS must extent.
The RBS checks to see if it is part of its OPTIMAL size.
RBS chooses its oldest inactive segment.
Oldest inactive segment is eliminated.
The data dictionary tables for space management are updated.
84) How can we plan storage for very large tables?
Ans: Limit the number of extents in the table
Separate table from its indexes.
Allocate sufficient temporary storage.
85) How will you estimate the space required by a non-clustered table?
Ans: Calculate the total header size
Calculate the available data space per data block
Calculate the combined column lengths of the average row
Calculate the total average row size.
Calculate the average number rows that can fit in a block
Calculate the number of blocks and bytes required for the table.
After arriving the calculation, add 10 % additional space to calculate the initial extent size for a working table.
86) It is possible to use raw devices as data files and what are the advantages over file system files?
The advantages over file system files are that I/O will be improved because Oracle is bye-passing the kernel which writing into disk. Disk corruption will be very less.
87) What is a Control file?
Ans: Database’s overall physical architecture is maintained in a file called control file. It will be used to maintain internal consistency and guide recovery operations. Multiple copies of control files are advisable.
88) How to implement the multiple control files for an existing database?
Ans: Shutdown the database
Copy one of the existing control file to new location
Edit Config ora file by adding new control filename
Restart the database.
89) What is redo log file mirroring? How can be achieved?
Ans: Process of having a copy of redo log files is called mirroring.
This can be achieved by creating group of log files together, so that LGWR will automatically writes them to all the members of the current on-line redo log group. If any one group fails then database automatically switch over to next group. It degrades performance.
90) What is advantage of having disk shadowing / mirroring?
Ans: Shadow set of disks save as a backup in the event of disk failure. In most operating systems if any disk failure occurs it automatically switchover to place of failed disk.
Improved performance because most OS support volume shadowing can direct file I/O request to use the shadow set of files instead of the main set of files. This reduces I/O load on the main set of disks.
91) What is use of rollback segments in Oracle database?
Ans: They allow the database to maintain read consistency between multiple transactions.
92) What is a rollback segment entry?
Ans: It is the set of before image data blocks that contain rows that are modified by a transaction.
Each rollback segment entry must be completed within one rollback segment.
A single rollback segment can have multiple rollback segment entries.
93) What is hit ratio?
Ans: It is a measure of well the data cache buffer is handling requests for data.
Hit Ratio = (Logical Reads – Physical Reads – Hits Misses)/ Logical Reads.
94) When will be a segment released?
Ans: When Segment is dropped.
When Shrink (RBS only)
When truncated (TRUNCATE used with drop storage option)
95) What are disadvantages of having raw devices?
Ans: We should depend on export/import utility for backup/recovery (fully reliable)
The tar command cannot be used for physical file backup, instead we can use dd command, which is less flexible and has limited recoveries.
96) List the factors that can affect the accuracy of the estimations?
– The space used transaction entries and deleted records, does not become free immediately after completion due to delayed cleanout.
– Trailing nulls and length bytes are not stored.
– Inserts of, updates to and deletes of rows as well as columns larger than a single data block, can cause fragmentation a chained row pieces.
Database Security & Administration
97) What is user Account in Oracle database?
Ans: A user account is not a physical structure in database but it is having important relationship to the objects in the database and will be having certain privileges.
98) How will you enforce security using stored procedures?
Ans: Don’t grant user access directly to tables within the application.
Instead grant the ability to access the procedures that access the tables.
When procedure executed it will execute the privilege of procedures owner. Users cannot access tables except via the procedure.
99) What are the dictionary tables used to monitor a database space?
100) What are the types of SQL statement?
Data Definition Language: CREATE, ALTER, DROP, TRUNCATE, REVOKE, NO AUDIT & COMMIT.
Data Manipulation Language: INSERT, UPDATE, DELETE, LOCK TABLE, EXPLAIN PLAN & SELECT.
Transactional Control: COMMIT & ROLLBACK
Session Control: ALTERSESSION & SET ROLE
System Control: ALTER SYSTEM.
101) What is a transaction?
Ans: Transaction is logical unit between two commits and commit and rollback.
102) What is difference between TRUNCATE & DELETE?
Ans: TRUNCATE commits after deleting entire table i.e., cannot be rolled back.
Database triggers do not fire on TRUNCATE
DELETE allows the filtered deletion. Deleted records can be rolled back or committed.
Database triggers fire on DELETE.
103) What is a join? Explain the different types of joins?
Ans: Join is a query, which retrieves related columns or rows from multiple tables.
Self Join – Joining the table with itself.
Equi Join – Joining two tables by equating two common columns.
Non-Equi Join – Joining two tables by equating two common columns.
Outer Join – Joining two tables in such a way that query can also retrieve rows that do not have corresponding join value in the other table.
104) What is the sub-query?
Ans: Sub-query is a query whose return values are used in filtering conditions of the main query.
105) What is correlated sub-query?
Ans: Correlated sub-query is a sub-query, which has reference to the main query.
106) Explain CONNECT BY PRIOR?
Ans: Retrieves rows in hierarchical order eg.
select empno, ename from emp where.
107) Difference between SUBSTR and INSTR?
Ans: INSTR (String1, String2 (n, (m)),
INSTR returns the position of the m-th occurrence of the string 2 in string1. The search begins from nth position of string1.
SUBSTR (String1 n, m)
SUBSTR returns a character string of size m in string1, starting from n-th position of string1.
108) Explain UNION, MINUS, UNION ALL and INTERSECT?
INTERSECT – returns all distinct rows selected by both queries.
MINUS – returns all distinct rows selected by the first query but not by the second.
UNION – returns all distinct rows selected by either query
UNION ALL – returns all rows selected by either query, including all duplicates.
109) What is ROWID?
Ans: ROWID is a pseudo column attached to each row of a table. It is 18 characters long, blockno, rownumber are the components of ROWID.
110) What is the fastest way of accessing a row in a table?
Ans: Using ROWID.
111) What is an integrity constraint?
Ans: Integrity constraint is a rule that restricts values to a column in a table.
112) What is referential integrity constraint?
Ans: Maintaining data integrity through a set of rules that restrict the values of one or more columns of the tables based on the values of primary key or unique key of the referenced table.
113) What is the usage of SAVEPOINTS?
Ans: SAVEPOINTS are used to subdivide a transaction into smaller parts. It enables rolling back part of a transaction. Maximum of five save points are allowed.
114) What is ON DELETE CASCADE?
Ans: When ON DELETE CASCADE is specified Oracle maintains referential integrity by automatically removing dependent foreign key values if a referenced primary or unique key value is removed.
115) What are the data types allowed in a table?
Ans: CHAR, VARCHAR2, NUMBER, DATE, RAW, LONG and LONG RAW.
116) What is difference between CHAR and VARCHAR2? What is the maximum SIZE allowed for each type?
CHAR pads blank spaces to the maximum length.
VARCHAR2 does not pad blank spaces.
For CHAR the maximum length is 255 and 2000 for VARCHAR2.
117) How many LONG columns are allowed in a table? Is it possible to use LONG columns in WHERE clause or ORDER BY?
Ans: Only one LONG column is allowed. It is not possible to use LONG column in WHERE or ORDER BY clause.
118) What are the pre-requisites to modify datatype of a column and to add a column with NOT NULL constraint?
– To modify the datatype of a column the column must be empty.
– To add a column with NOT NULL constrain, the table must be empty.
119) Where the integrity constraints are stored in data dictionary?
Ans: The integrity constraints are stored in USER_CONSTRAINTS.
120) How will you activate/deactivate integrity constraints?
Ans: The integrity constraints can be enabled or disabled by ALTER TABLE ENABLE CONSTRAINT / DISABLE CONSTRAINT.
121) If unique key constraint on DATE column is created, will it validate the rows that are inserted with SYSDATE?
Ans: It won’t, Because SYSDATE format contains time attached with it.
122) What is a database link?
Ans: Database link is a named path through which a remote database can be accessed.
123) How to access the current value and next value from a sequence? Is it possible to access the current value in a session before accessing next value?
Ans: Sequence name CURRVAL, sequence name NEXTVAL. It is not possible. Only if you access next value in the session, the current value can be accessed.
124) What is CYCLE/NO CYCLE in a Sequence?
Ans: CYCLE specifies that the sequence continues to generate values after reaching either maximum or minimum value. After pan-ascending sequence reaches its maximum value, it generates its minimum value. After a descending sequence reaches its minimum, it generates its maximum.
NO CYCLE specifies that the sequence cannot generate more values after reaching its maximum or minimum value.
125) What are the advantages of VIEW?
– To protect some of the columns of a table from other users.
– To hide the complexity of a query.
– To hide the complexity of calculations.
126) Can a view be updated/inserted/deleted? If Yes – under what conditions?
Ans: A View can be updated/deleted/inserted if it has only one base table if the view is based on columns from one or more tables then insert, update and delete is not possible.
127) If a view on a single base table is manipulated will the changes be reflected in the base table?
Ans: If changes are made to the tables and these tables are the base tables of a view, then the changes will be referenced in the view.
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