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mysql interview questions

Top mysql frequently asked interview questions

How to find all the tables in MySQL with specific column names in them?

I have 2-3 different column names that I want to look up in the entire DB and list out all tables which have those columns. Any easy script?


Source: (StackOverflow)

Would you recommend PostgreSQL over MySQL? [closed]

We are currently working with JavaEE and MySQL 5 in our company, but we have some queries, especially delete queries that take > 10 min to execute. We are considering a switch to PostgreSQL.

What are the advantages of PostgreSQL over MySQL, if there are any? Do you have experience with both DBs and give me a roundup about whether this is a good idea or does this fully depend on the needs of our server?


Source: (StackOverflow)

Installing specific package versions with pip

I'm trying to install version 1.2.2 of the MySQL_python adaptor. The current version shown in PyPi is 1.2.3. Is there a way to install the older version? I found an article stating that this should do it:

pip install MySQL_python==1.2.2

When installed, however, it still shows MySQL_python-1.2.3-py2.6.egg-info in the site packages. Is this a problem specific to this package, or am I doing something wrong?


Source: (StackOverflow)

How to shrink/purge ibdata1 file in MySQL

I am using MySQL in localhost as a "query tool" for performing statistics in R, that is, everytime I run a R script, I create a new database (A), create a new table (B), import the data into B, submit a query to get what I need, and then I drop B and drop A.

It's working fine for me, but I realize that the ibdata file size is increasing rapidly, I stored nothing in MySQL, but the ibdata1 file already exceeded 100 MB.

I am using more or less default MySQL setting for the setup, is there a way for I can automatically shrink/purge the ibdata1 file after a fixed period of time?


Source: (StackOverflow)

How can I SELECT rows with MAX(Column value), DISTINCT by another column in SQL?

My table is:

id  home  datetime     player   resource
---|-----|------------|--------|---------
1  | 10  | 04/03/2009 | john   | 399 
2  | 11  | 04/03/2009 | juliet | 244
5  | 12  | 04/03/2009 | borat  | 555
3  | 10  | 03/03/2009 | john   | 300
4  | 11  | 03/03/2009 | juliet | 200
6  | 12  | 03/03/2009 | borat  | 500
7  | 13  | 24/12/2008 | borat  | 600
8  | 13  | 01/01/2009 | borat  | 700

I need to select each distinct home holding the maximum value of datetime.

Result would be:

id  home  datetime     player   resource 
---|-----|------------|--------|---------
1  | 10  | 04/03/2009 | john   | 399
2  | 11  | 04/03/2009 | juliet | 244
5  | 12  | 04/03/2009 | borat  | 555
8  | 13  | 01/01/2009 | borat  | 700

I have tried:

-- 1 ..by the MySQL manual: 

SELECT DISTINCT home, id, datetime as dt, player, resource
    FROM topten t1
    WHERE datetime = (SELECT MAX(t2.datetime) FROM topten t2
        GROUP BY home )
GROUP BY datetime
ORDER BY datetime DESC

Doesn't work. Result-set has 130 rows although database holds 187. Result includes some duplicates of home.

-- 2 ..join

SELECT s1.id, s1.home, s1.datetime, s1.player, s1.resource
FROM topten s1 JOIN
(SELECT id, MAX(datetime) AS dt
  FROM topten
  GROUP BY id) AS s2
  ON s1.id = s2.id
  ORDER BY datetime 

Nope. Gives all the records.

-- 3 ..something exotic: 

With various results.


Source: (StackOverflow)

INNER JOIN ON vs WHERE clause

For simplicity, assume all relevant fields are NOT NULL.

You can do:

SELECT
    table1.this, table2.that, table2.somethingelse
FROM
    table1, table2
WHERE
    table1.foreignkey = table2.primarykey
    AND (some other conditions)

Or else:

SELECT
    table1.this, table2.that, table2.somethingelse
FROM
    table1 INNER JOIN table2
    ON table1.foreignkey = table2.primarykey
WHERE
    (some other conditions)

Are those two worked on the same way by MySQL?


Source: (StackOverflow)

How do I specify unique constraint for multiple columns in MySQL?

I have a table:

table votes (
    id,
    user,
    email,
    address,
    primary key(id),
);

Now I want to make the columns user, email, address unique (together).

How do I do this in MySql?

  • Of course the example is just... an example. So please don't worry about the semantics.

Source: (StackOverflow)

SQL Select only rows with Max Value on a Column

I have this table for documents (simplified version here):

+------+-------+--------------------------------------+
| id   | rev   | content                              |
+------+-------+--------------------------------------+
| 1    | 1     | ...                                  |
| 2    | 1     | ...                                  |
| 1    | 2     | ...                                  |
| 1    | 3     | ...                                  |
+------+-------+--------------------------------------+

How do I select one row per id and only the greatest rev?
With the above data, the result should contain two rows: [1, 3, ...] and [2, 1, ..]. I'm using MySQL.

Currently I use checks in the while loop to detect and over-write old revs from the resultset. But is this the only method to achieve the result? Isn't there a SQL solution?

Update
As the answers suggest, there is a SQL solution, and here a sqlfiddle demo.

Update 2
I noticed after adding the above sqlfiddle, the rate at which the question is upvoted has surpassed the upvote rate of the answers. That has not been the intention! The fiddle is based on the answers, especially the accepted answer.


Source: (StackOverflow)

How to 'insert if not exists' in MySQL?

I started by googling, and found this article which talks about mutex tables.

I have a table with ~14 million records. If I want to add more data in the same format, is there a way to ensure the record I want to insert does not already exist without using a pair of queries (ie, one query to check and one to insert is the result set is empty)?

Does a unique constraint on a field guarantee the insert will fail if it's already there?

It seems that with merely a constraint, when I issue the insert via php, the script croaks.


Source: (StackOverflow)

How do I quickly rename a MySQL database (change schema name)?

The MySQL manual at MySQL covers this.

Usually I just dump the database and reimport it with a new name. This is not an option for very big databases. Apparently RENAME {DATABASE | SCHEMA} db_name TO new_db_name; does bad things, exist only in a handful of versions, and is a bad idea overall.

This needs to work with InnoDB, which stores things very differently than MyISAM.


Source: (StackOverflow)

"INSERT IGNORE" vs "INSERT ... ON DUPLICATE KEY UPDATE"

While executing an INSERT statement with many rows, I want to skip duplicate entries that would otherwise cause failure. After some research, my options appear to be the use of either:

  • ON DUPLICATE KEY UPDATE which implies an unnecessary update at some cost, or
  • INSERT IGNORE which implies an invitation for other kinds of failure to slip in unannounced.

Am I right in these assumptions? What's the best way to simply skip the rows that might cause duplicates and just continue on to the other rows?


Source: (StackOverflow)

How do you set a default value for a MySQL Datetime column?

How do you set a default value for a MySQL Datetime column?

In SQL Server it's getdate(). What is the equivalant for MySQL? I'm using MySQL 5.x if that is a factor.


Source: (StackOverflow)

MySQL: Large VARCHAR vs. TEXT?

I've got a messages table in MySQL which records messages between users. Apart from the typical ids and message types (all integer types) I need to save the actual message text as either VARCHAR or TEXT. I'm setting a front-end limit of 3000 characters which means the messages would never be inserted into the db as longer than this.

Is there a rationale for going with either VARCHAR(3000) or TEXT? There's something about just writing VARCHAR(3000) that feels somewhat counter-intuitive. I've been through other similar posts on Stack Overflow but would be good to get views specific to this type of common message storing.

Any help would be appreciated. Thanks.


Source: (StackOverflow)

What's the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN? [duplicate]

This question already has an answer here:

What's the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN in MySQL?


Source: (StackOverflow)

Can I concatenate multiple MySQL rows into one field?

Using MySQL, I can do something like:

SELECT hobbies FROM peoples_hobbies WHERE person_id = 5;

and get:

shopping
fishing
coding

but instead I just want 1 row, 1 col:

shopping, fishing, coding

The reason is that I'm selecting multiple values from multiple tables, and after all the joins I've got a lot more rows than I'd like.

I've looked for a function on MySQL Doc and it doesn't look like the CONCAT or CONCAT_WS functions accept result sets, so does anyone here know how to do this?


Source: (StackOverflow)