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MS SQL Interview Questions Answers

MS SQL Interview Questions Answers

Question - 1 : - Getting Maximum Performance from MySQL ?

Answer - 1 : - Optimization is a complicated task because it ultimately requires understanding of the whole system. While it may be possible to do some local optimizations with small knowledge of your system/application, the more optimal you want your system to become the more you will have to know about it. So this chapter will try to explain and give some examples of different ways to optimize MySQL. But remember that there are always some (increasingly harder) additional ways to make the system even faster.

Question - 2 : - Explain federated tables. ?

Answer - 2 : - Introduced in MySQL 5.0, federated tables allow access to the tables located on other databases on other servers.

Question - 3 : - How do you add three minutes to a date?

Answer - 3 : - ADDDATE(techpreparation_publication_date, INTERVAL 3 MINUTE)

Question - 4 : - How do I retrieve only the date or time portion of the data?

Answer - 4 : - You can use T-SQL's DATEPART() function to return a subset of the values that SQL Server's datetime columns store. The DATEPART() function uses two arguments. The first argument specifies the portion of the date that you want, and the second value specifies the datetime column: SELECT orderID, DATEPART(MM,OrderDate) AS OrderMonth FROM Orders

Question - 5 : - Speed of SELECT Queries ?

Answer - 5 : - In general, when you want to make a slow SELECT ... WHERE faster, the first thing to check is whether or not you can add an index. All references between different tables should usually be done with indexes. You can use the EXPLAIN command to determine which indexes are used for a SELECT. Some general tips: To help MySQL optimize queries better, run myisamchk --analyze on a table after it has been loaded with relevant data. This updates a value for each index part that indicates the average number of rows that have the same value. (For unique indexes, this is always 1, of course.). MySQL will use this to decide which index to choose when you connect two tables with 'a non-constant expression'. You can check the result from the analyze run by doing SHOW INDEX FROM table_name and examining the Cardinality column. To sort an index and data according to an index, use myisamchk --sort-index --sort-records=1 (if you want to sort on index 1). If you have a unique index from which you want to read all records in order according to that index, this is a good way to make that faster. Note, however, that this sorting isn't written optimally and will take a long time for a large table!

Question - 6 : - What the Privilege System Does?

Answer - 6 : - The primary function of the MySQL privilege system is to authenticate a user connecting from a given host, and to associate that user with privileges on a database such as select, insert, update and delete. Additional functionality includes the ability to have an anonymous user and to grant privileges for MySQL-specific functions such as LOAD DATA INFILE and administrative operations. MySQL User Names and Passwords There are several distinctions between the way user names and passwords are used by MySQL and the way they are used by Unix or Windows: User names, as used by MySQL for authentication purposes, have nothing to do with Unix user names (login names) or Windows user names. Most MySQL clients by default try to log in using the current Unix user name as the MySQL user name, but that is for convenience only. Client programs allow a different name to be specified with the -u or --user options. This means that you can't make a database secure in any way unless all MySQL user names have passwords. Anyone may attempt to connect to the server using any name, and they will succeed if they specify any name that doesn't have a password. MySQL user names can be up to 16 characters long; Unix user names typically are limited to 8 characters. MySQL passwords have nothing to do with Unix passwords. There is no necessary connection between the password you use to log in to a Unix machine and the password you use to access a database on that machine. MySQL encrypts passwords using a different algorithm than the one used during the Unix login process. Note that even if the password is stored 'scrambled', and knowing your 'scrambled' password is enough to be able to connect to the MySQL server! Connecting to the MySQL Server MySQL client programs generally require that you specify connection parameters when you want to access a MySQL server: the host you want to connect to, your user name, and your password. For example, the mysql client can be started like this (optional arguments are enclosed between `[' and `]'): shell> mysql [-h host_name] [-u user_name] [-pyour_pass] Alternate forms of the -h, -u, and -p options are --host=host_name, --user=user_name, and --password=your_pass. Note that there is no space between -p or --password= and the password following

Question - 7 : - If you specify the data type as DECIMAL (5,2), what’s the range of values that can go in this table?

Answer - 7 : - 999.99 to -99.99. Note that with the negative number the minus sign is considered one of the digits.

Question - 8 : - Explain advantages of MyISAM over InnoDB?

Answer - 8 : - Much more conservative approach to disk space management - each MyISAM table is stored in a separate file, which could be compressed then with myisamchk if needed. With InnoDB the tables are stored in tablespace, and not much further optimization is possible. All data except for TEXT and BLOB can occupy 8,000 bytes at most. No full text indexing is available for InnoDB. TRhe COUNT(*)s execute slower than in MyISAM due to tablespace complexity.

Question - 9 : - How can you ensure that a table named TableB with a field named Fld1 will only have those values in the Fld1 field that are also in the table named TableA with a field named Fld1?

Answer - 9 : - This relationship related question has two potential answers. The first answer (and the one that you want to hear) is the use of foreign key constraints. A foreign key constraint is used to maintain referential integrity. It is used to ensure that a field in a table will only hold values that are already defined in another field in a different (or the same) table. That field is the candidate key (usually a primary key of the other table). The other option is the use of triggers. Triggers can be used to ensure the same effect of constraints in a roundabout way, but it is much more difficult to set up and maintain, and the performance is typically worse. Because of this, Microsoft recommends that developers use foreign key constraints instead of triggers for maintaining referential integrity.

Question - 10 : - Quries Other Optimization Tips

Answer - 10 : - Use persistent connections to the database to avoid the connection overhead. If you can't use persistent connections and you are doing a lot of new connections to the database, you may want to change the value of the thread_cache_size variable. Always check that all your queries really use the indexes you have created in the tables. In MySQL you can do this with the EXPLAIN command. Try to avoid complex SELECT queries on tables that are updated a lot. This is to avoid problems with table locking. The new MyISAM tables can insert rows in a table without deleted rows at the same time another table is reading from it. If this is important for you, you should consider methods where you don't have to delete rows or run OPTIMIZE TABLE after you have deleted a lot of rows. Use ALTER TABLE ... ORDER BY expr1,expr2... if you mostly retrieve rows in expr1,expr2.. order. By using this option after big changes to the table, you may be able to get higher performance. In some cases it may make sense to introduce a column that is 'hashed' based on information from other columns. If this column is short and reasonably unique it may be much faster than a big index on many columns. In MySQL it's very easy to use this extra column: SELECT * FROM table_name WHERE hash=MD5(concat(col1,col2)) AND col_1='constant' AND col_2='constant' For tables that change a lot you should try to avoid all VARCHAR or BLOB columns. You will get dynamic row length as soon as you are using a single VARCHAR or BLOB column. It's not normally useful to split a table into different tables just because the rows gets 'big'. To access a row, the biggest performance hit is the disk seek to find the first byte of the row. After finding the data most new disks can read the whole row fast enough for most applications. The only cases where it really matters to split up a table is if it's a dynamic row size table (see above) that you can change to a fixed row size, or if you very often need to scan the table and don't need most of the columns. If you very often need to calculate things based on information from a lot of rows (like counts of things), it's probably much better to introduce a new table and update the counter in real time. An update of type UPDATE table set count=count+1 where index_column=constant is very fast! This is really important when you use databases like MySQL that only have table locking (multiple readers /


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