Answer:

To show the current privileges of a MySQL user, you can use the SHOW GRANTS statement

E.g.

mysql> SHOW GRANTS;

+----------------------------------------------------------------------------------------------------------------------------------------+
| Grants for root@localhost                                                                                                              |
+----------------------------------------------------------------------------------------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost' IDENTIFIED BY PASSWORD '*****************************************' WITH GRANT OPTION |
+----------------------------------------------------------------------------------------------------------------------------------------+

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Answer:

Like in the shell, we can execute the command whoami to find out the current user name, we can use the function CURRENT_USER();

E.g.

mysql> SELECT CURRENT_USER();
+----------------+
| CURRENT_USER() |
+----------------+
| root@localhost |
+----------------+
1 row in set (0.00 sec)

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Answer:

To select (find) a document in MongoDB, you can use the following syntax in the mongodb Interactive shell.

use my_db
db.users.find({name:"peter"})

You need to switch to the current database (e.g. my_db) and then execute your select statement.

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Answer:

Like MySQL, you can get a better understanding of the performance of your MongoDB's query, you can use the $explain feature.

E.g.

db.collection.find(query).explain();

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Answer:

If you only connect to the MySQL server from localhost and you might want to disable TCP/IP networking feature so the server is more secure.

To do so, edit the MySQL configurations, e.g. /etc/my.cnf

..
skip-networking
..

Don't forget to restart MySQL to take effect.

# /sbin/service mysqld restart

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Answer:

To repair and compact a database in MongoDB, you can use the following command in the mongodb Interactive shell.

1. Select the database

use test

2. Repair the current database

db.repairDatabase()

Done.

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Answer:

In MongoDB, to count the total number of objects in the collection, you can use the following statement in the mongodb Interactive shell.

db.test.count()

The above statement will return the total number of objects in the collection "test"

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Answer:

In an given collection, if you want to add an index on a specify field, you can do the following in the mongodb Interactive shell.

db.test.ensureIndex( { name : 1 } )

The key "name" from now on will have index enabled.

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2. Repair and compact database in MongoDB
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Answer:

In previous article, we have already discussed how to list the database size in your MySQL server.

But if you want to have a more detail view on an individual database, you can try the following (crazy) SQL.

SELECT s.schema_name,
CONCAT(IFNULL(ROUND((SUM(t.data_length)+SUM(t.index_length))
/1024/1024,2),0.00),"Mb") total_size,
CONCAT(IFNULL(ROUND(((SUM(t.data_length)+SUM(t.index_length))-SUM(t.data_free))/1024/1024,2),0.00),"Mb")

data_used,
CONCAT(IFNULL(ROUND(SUM(data_free)/1024/1024,2),0.00),"Mb") data_free,
IFNULL(ROUND((((SUM(t.data_length)+SUM(t.index_length))-SUM(t.data_free))
/((SUM(t.data_length)+SUM(t.index_length)))*100),2),0) pct_used,
COUNT(table_name) total_tables
FROM INFORMATION_SCHEMA.SCHEMATA s
LEFT JOIN INFORMATION_SCHEMA.TABLES t ON s.schema_name = t.table_schema
WHERE s.schema_name = "mydatabase"
GROUP BY s.schema_name
ORDER BY pct_used DESC\G

It will give you something like the following..

 schema_name: mydatabase
  total_size: 165.09Mb
   data_used: 12.09Mb
   data_free: 153.00Mb
    pct_used: 7.33
total_tables: 133

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Answer:

In an given collection, if you want to perform update on an existing document, the easiest way is to use the following commands.

employee = db.employees.findOne( { name : "Peter" } );
employee.position = "Project Manager";
db.employees.save( employee );

Done.

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