Shantanu's Blog

Corporate Consultant

October 02, 2007

 

MySQL FAQ - 3

1) Importing and exporting Excel Data

a) The syntax to import the CSV data from text file is simple and is very fast.

LOAD DATA INFILE 'datafile.txt' INTO TABLE employee
(employee_number,firstname,surname,tel_no,salary) FIELDS TERMINATED BY '|'");

LOAD DATA INFILE has defaults of:
FIELDS TERMINATED BY '\t' ENCLOSED BY '' ESCAPED BY '\\'

b) The trick to export the data in CSV or TSV format is to add the into outfile line like this...

set @row:= 0;
(select 'srno', 'enroll_no', 'stud_fname', 'stud_lname', 'stud_address', 'stud_address1', 'stud_address2', 'stud_city', 'stud_pin', 'state_nm')
union
(select (@row:= @row + 1) as srno, a.enroll_no, stud_fname, stud_lname, stud_address
into outfile '/home/shantanu/CAT_ADV_OCT_07.tsv' FIELDS TERMINATED BY '|' LINES TERMINATED BY '\n'
from course_enroll as a,
course as b,
where date like '%2007%' and flag in ('o','a') and
a.branch_id=b.branch_id
group by a.enroll_no
order by stud_pin);
_____

Here is another way to import / export data from Excel

a) In order to import data from excel to MySQL here are 2 easy steps.
Select Text (MS-DOS) as an option in "Save as type" drop down while saving the file. Use the following command to load the data in MySQL.

load data local infile 'country_code.txt' into table test.country_code columns optionally enclosed by '"';

You may need to trim the data if extra spaces are added to the beginning or end of the string.

update country_code set iso_code = trim(iso_code);

_____

b) To export the data, use the standard and in and standard out descriptors as this...

1) Save the query in a text file. For e.g. query.txt

2) Use the following command to generate the excel readable result file.
mysql country_list < query.txt > query_to_excel.txt


2) Delete V/s truncate

a) Do NOT use:

DELETE FROM classifieds;

Rather, use:

TRUNCATE TABLE classifieds;

The difference here is that DELETE drops records one by one, and that can be 1 million one by one's too slow!

b) If you want the count of records those were deleted then you have to use delete from command like this...
DELETE FROM classifieds where 1 = 1
It will display XXX records deleted message once it completes the operation.



3) Analyze and Optimize tables:

You can provide information to the parser by running

ANALYZE TABLE tablename;

This stores the key distribution for the table (running ANALYZE is equivalent to running myisamchk -a or myismachk --analyze). Many deletes and updates leave gaps in the table (especially when you're using varchar, or in particular text/blob fields). This means there are more unnecessary disk I/O's, as the head needs to skip over these gaps when reading. Running

OPTIMIZE TABLE tablename

solves this problem. Both of these statements should be run fairly frequently in any well looked after system.


4) Explain and Procedure:

a) Add the word EXPLAIN before any SELECT statement to know the "kundali" of the command.

EXPLAIN SELECT firstname FROM employee WHERE overtime_rate<20*2;

+--------+-------+---------------+---------------+---------+------+------+----------+
|table | TYPE | possible_keys | key | key_len | ref | rows |Extra |
+--------+-------+---------------+---------------+---------+------+------+----------+
|employee| range | overtime_rate | overtime_rate | 4 | NULL | 1 |where used|
+--------+-------+---------------+---------------+---------+------+------+----------+

The output from EXPLAIN shows "ALL" in the TYPE column when MySQL uses a table scan to resolve a query. The possible types are, from best to worst: system, const, eq_ref, ref, range, index and ALL. MySQL can perform the 20*2 calculation once, and then search the index for this constant.

explain tbl_name
will display all the relevant info about the table in question.

SHOW TABLE STATUS LIKE 'your_table_name‘
To find the time stamp of table creation.

b) Explain provides more information about indexes, but procedure analyse() gives you more information on data returned.

SELECT center_code FROM employee PROCEDURE ANALYSE()

Min_value : 34
Max_value : 232
Empties_or_zeros : 0
Nulls : 0
Avg_value : 133
Optimal_fieldtype : ENUM('34','232') NOT NULL


5) SHOW commands:

SHOW PROCESSLIST
// you can just check if your query running, or is waiting for some lock.
SHOW FULL PROCESSLIST
// to see which query or set of queries take the longest time

SHOW CREATE TABLE employee
// display exactly how the table was created.

DESCRIBE tbl_name
DESC tbl_name
// just like show create table command

SHOW VARIABLES
// All the information to know the state of server status

STATUS
// Information on connection, uptime, version and user


6) Indexes on partial columns:
In the last post, I discussed composite indexes. But there is a limit of 556 bytes those can be grouped together in a single index. So the following statement would fail if each of the fields is declared as 250 characters since the total will be more than 556.

ALTER TABLE employee ADD INDEX(surname, firstname, middlename);

Instead we can index on partial text something like this...

ALTER TABLE employee ADD INDEX(surname(20),firstname(20), middlename(20));

Now our updates write to an index are just 10% of the original and it will accept 3 column composite index even if the field total is more than 556.

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