Question
1
:
what are the features of MySQL ?
Answer
1 :
The technical features of MySQL
For advanced technical information, see section 7 MySQL Language Reference. MySQL is a client/server system that consists of a multi-threaded SQL server that supports different backends, several different client programs and libraries, administrative tools, and a programming interface. We also provide MySQL as a multi-threaded library which you can link into your application to get a smaller, faster, easier to manage product. MySQL has a lot of contributed software available.
It is very likely that you will find that your favorite application/language already supports MySQL. The official way to pronounce MySQL is ``My Ess Que Ell'' (not MY-SEQUEL). But we try to avoid correcting people who say MY-SEQUEL.
The Main Features of MySQL
The following list describes some of the important characteristics of MySQL:
Fully multi-threaded using kernel threads. That means it can easily use multiple CPUs if available.
C, C++, Eiffel, Java, Perl, PHP, Python and Tcl APIs.
Works on many different platforms.
Many column types: signed/unsigned integers 1, 2, 3, 4, and 8 bytes long, FLOAT, DOUBLE, CHAR, VARCHAR, TEXT, BLOB, DATE, TIME, DATETIME, TIMESTAMP, YEAR, SET, and ENUM types.
Very fast joins using an optimized one-sweep multi-join.
Full operator and function support in the SELECT and WHERE parts of queries. Example:
mysql> SELECT CONCAT(first_name, " ", last_name) FROM tbl_name
WHERE income/dependents > 10000 AND age > 30;
SQL functions are implemented through a highly optimized class library and should be as fast as they can get! Usually there shouldn't be any memory allocation at all after query initialization.
Full support for SQL GROUP BY and ORDER BY clauses. Support for group functions (COUNT(), COUNT(DISTINCT), AVG(), STD(), SUM(), MAX() and MIN()).
Support for LEFT OUTER JOIN and RIGHT OUTER JOIN with ANSI SQL and ODBC syntax.
You can mix tables from different databases in the same query (as of Version 3.22).
A privilege and password system that is very flexible and secure and allows host-based verification. Passwords are secure because all password traffic is encrypted when you connect to a server.
ODBC (Open-DataBase-Connectivity) support for Win32 (with source). All ODBC 2.5 functions and many others. For example, you can use MS Acce
Question
2
:
Quries Other Optimization Tips
Answer
2 :
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 /
Question
3
:
If you run this, what does it return?
select applock_mode('public', 'SalesApp', 'Transaction')
Answer
3 :
The type of lock being held by an application that requested it.
This command returns the lock mode held by an application that was requested with the sp_getapplock procedure.
insert mytable select ''
insert mytable select ' '
select * from mytable where mychar = ''
select * from mytable where mychar = ' '
Question
4
:
What are a primary and foreign key?
Answer
4 :
A primary key is the field(s) in a table that uniquely defines the row in the table; the values in the primary key are always unique. A foreign key is a constraint that establishes a relationship between two tables. This relationship typically involves the primary key field(s) from one table with an adjoining set of field(s) in another table (although it could be the same table). The adjoining field(s) is the foreign key.
Question
5
:
How many Service Packs will be released for SQL Server 2005 in 2007?
Answer
5 :
The answer is up in the air and this is more of a poll than a real QOD. Based on the ways things are going, the staff here sees just 1, though our hope would be that 3 or 4 would be released.
You setup a linked server from a SQL Server 2000 server to your new SQL Server 2005 server (with defaults), however you cannot execute
Question
6
:
When you create a table, and then run SHOW CREATE TABLE on it, you occasionally get different results than what you typed in. What does MySQL modify in your newly created tables?
Answer
6 :
1. VARCHARs with length less than 4 become CHARs
2. CHARs with length more than 3 become VARCHARs.
3. NOT NULL gets added to the columns declared as PRIMARY KEYs
4. Default values such as NULL are specified for each column
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
:
How MySQL Optimizes LEFT JOIN and RIGHT JOIN ?
Answer
8 :
A LEFT JOIN B in MySQL is implemented as follows:
The table B is set to be dependent on table A and all tables that A is dependent on.
The table A is set to be dependent on all tables (except B) that are used in the LEFT JOIN condition.
All LEFT JOIN conditions are moved to the WHERE clause.
All standard join optimizations are done, with the exception that a table is always read after all tables it is dependent on. If there is a circular dependence then MySQL will issue an error.
All standard WHERE optimizations are done.
If there is a row in A that matches the WHERE clause, but there wasn't any row in B that matched the LEFT JOIN condition, then an extra B row is generated with all columns set to NULL.
If you use LEFT JOIN to find rows that don't exist in some table and you have the following test: column_name IS NULL in the WHERE part, where column_name is a column that is declared as NOT NULL, then MySQL will stop searching after more rows (for a particular key combination) after it has found one row that matches the LEFT JOIN condition.
RIGHT JOIN is implemented analogously as LEFT JOIN.
The table read order forced by LEFT JOIN and STRAIGHT JOIN will help the join optimizer (which calculates in which order tables should be joined) to do its work much more quickly, as there are fewer table permutations to check.
Note that the above means that if you do a query of type:
SELECT * FROM a,b LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key) WHERE b.key=d.key
MySQL will do a full scan on b as the LEFT JOIN will force it to be read before d.
The fix in this case is to change the query to:
SELECT * FROM b,a LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key) WHERE b.key=d.key
Question
9
:
How do you add three minutes to a date?
Answer
9 :
ADDDATE(techpreparation_publication_date, INTERVAL 3 MINUTE)
Question
10
:
How would you delete a column?
Answer
10 :
ALTER TABLE techpreparation_answers DROP answer_user_id.