# Querying and fetching data¶

Now that we covered how to insert data into the table, you might be wondering how to retreive it. In this section, we will explore not only how to fetch the data, but how to narrow down and only fetch the data you want. But first let’s start with the simplest case - let’s fetch all the data in the table.

## Fetching all the data¶

You can fetch all the data stored in a table by simply calling the fetch method on the table object:

data = mouse.fetch()    # this retrieves all data in the table


By default the data is retrieved as numpy structured array:

>>> data
array([(  0, datetime.date(2017, 3, 1), 'M'),
(  1, datetime.date(2016, 11, 19), 'M'),
(  2, datetime.date(2016, 11, 20), 'U'),
(  5, datetime.date(2016, 12, 25), 'F'),
( 10, datetime.date(2017, 1, 1), 'F'),
( 11, datetime.date(2017, 1, 3), 'F'),
(100, datetime.date(2017, 5, 12), 'F')],
dtype=[('mouse_id', '<i8'), ('dob', 'O'), ('sex', 'O')])


Alternatively, you can retrieve the data as a list of dictionaries:

 >>> mouse.fetch(as_dict=True)   # retrieve all data as list of dictionaries
[OrderedDict([('mouse_id', 0),
('dob', datetime.date(2017, 3, 1)),
('sex', 'M')]),
OrderedDict([('mouse_id', 1),
('dob', datetime.date(2016, 11, 19)),
('sex', 'M')]),

...output truncated...

OrderedDict([('mouse_id', 100),
('dob', datetime.date(2017, 5, 12)),
('sex', 'F')])]


## Fetching specific attributes¶

If you are only interested in values from a specific attribute (columns) of the table, you can do so by passing in the name of the attribute(s) into the fetch method. Let’s try fetching only the date of birth of all mice.

>>> mouse.fetch('dob')
array([datetime.date(2017, 3, 1), datetime.date(2016, 11, 19),
datetime.date(2016, 11, 20), datetime.date(2016, 12, 25),
datetime.date(2017, 1, 1), datetime.date(2017, 1, 3),
datetime.date(2017, 5, 12)], dtype=object)


This can also be used to fetch multiple attributes separately:

>>> sex, ids = mouse.fetch('sex', 'mouse_id')
>>> sex
array(['M', 'M', 'U', 'F', 'F', 'F', 'F'], dtype=object)
>>> dob
array([  0,   1,   2,   5,  10,  11, 100])


Now that you have seen how to insert and then fetch data from a table, you could already start to use tables to store simple data. However, the true power of DataJoint comes about when you start combining these operations with DataJoint’s simple yet very powerful query language.

## Querying Data¶

The process of querying data refers to the searching and narrowing down of the existing data to find exactly what you need. Rather than retrieving all data and then writing your own parsing function to extract the data you want, you can use DataJoint’s query language to narrow things down first and then only retrieve what you need. Let’s take a look, using our Mouse table as the example.

At the moment, the Mouse table contains several entries:

>>> mouse
*mouse_id    dob            sex
+----------+ +------------+ +--------+
0            2017-03-01     M
1            2016-11-19     M
2            2016-11-20     U
5            2016-12-25     F
10           2017-01-01     F
11           2017-01-03     F
100          2017-05-12     F
(7 tuples)


Note

Before moving on, feel free to add more entries into your table using any one of insert methods that was covered in Populating the table.

### Restricting by attribute value¶

Let’s start with a very simple query, looking for an entry with a specific value of an attribute. We can find information about the mouse with mouse_id = 0 as follows:

>>> mouse & 'mouse_id = 0'
*mouse_id    dob            sex
+----------+ +------------+ +--------+
0            2017-03-01     M
(1 tuples)


Let’s take a closer look at what just happened. Using the table instance mouse, we used the & (restriction) operation to restrict down to entries that matches the restriction mouse_id = 0. Since there is only one mouse with mouse_id = 0 (recall that mouse_id is the primary key), we get back only one entry.

Now, let’s say we want to list only male mice. This is easily done by:

>>> mouse & 'sex = "M"'
*mouse_id    dob            sex
+----------+ +------------+ +--------+
0            2017-03-01     M
1            2016-11-19     M
(2 tuples)


Notice that "M" was surrounded by double quotes (") because the value was non-numeric.

### Using inequality in restriction¶

We can also use inequalities in our query, for eample to search for all mice born after Jan 1, 2017:

>>> mouse & 'dob > "2017-01-01"'
*mouse_id    dob            sex
+----------+ +------------+ +--------+
0            2017-03-01     M
11           2017-01-03     F
100          2017-05-12     F
(3 tuples)


Or you can find all mice that are not male:

>>> mouse & 'sex != "M"'
*mouse_id    dob            sex
+----------+ +------------+ +--------+
2            2016-11-20     U
5            2016-12-25     F
10           2017-01-01     F
11           2017-01-03     F
100          2017-05-12     F
(5 tuples)


### Combining restrictions¶

You can also combine multiple restrictions to form more complex queries:

>>> mouse & 'dob > "2017-01-01"' & 'sex = "M"'  # all male mice born after Jan 1, 2017
*mouse_id    dob            sex
+----------+ +------------+ +--------+
0            2017-03-01     M
(1 tuples)


### Restricting by a dictionary¶

If you are only looking for an equivalence of attribute values (i.e. you only need to use = in the restriction), you can also use a dictionary to restrict.

For example, the earlier query:

>>> mouse & 'sex = "M"'
*mouse_id    dob            sex
+----------+ +------------+ +--------+
0            2017-03-01     M
1            2016-11-19     M
(2 tuples)


can also be achieved using a dictionary as follows:

>>> r = {
'sex': 'M'
}
>>> mouse & r
*mouse_id    dob            sex
+----------+ +------------+ +--------+
0            2017-03-01     M
1            2016-11-19     M
(2 tuples)


Restricting with a dictionary comes in handy when you want to restrict by multiple attribute values.

## Fetching query results¶

Once you are happy with your query, you can fetch only the matching entries by calling fetch on the query result:

>>> (mouse & 'dob > "2017-01-01"' & 'sex = "M"').fetch()
array([(0, datetime.date(2017, 3, 1), 'M')],
dtype=[('mouse_id', '<i8'), ('dob', 'O'), ('sex', 'O')])


Not only does querying with DataJoint makes retrieving certain subsets of data easier, it also helps you avoid unnecessary data transfers between the database server and your computer. While you are forming and previewing queries, the query processing is actually performed by the database server, and only minimal data (if any) is transferred between the database server and your computer.

When you call fetch on the query result, only the relevant data is transfered, thus potentially cutting down the amount of data that has to be transferred out of the database server to your local machine. Not only does it save space on your machine, but can also significantly reduce data transfer speed and also help to reduce load on the database server.

## Deleting entires¶

Now that we have learned how to restrict our selection from a table, it’s an appropriate time for us to learn how to delete entries. As you might have guessed, you can delete entries by calling the delete method:

>>> mouse.delete()      # this will attept to delete ALL entries
The contents of the following tables are about to be deleted:
dj_tutorial.mouse (7 tuples)
Proceed? [yes, No]: no


Either hit enter without typing anything or type in “no” to cancel the deletion. If you can only delete all entries, then this would not be too useful. Fortunately, you can delete restricted table. For example, if I want to specifically delete mouse with ID of 0:

>>> (mouse & 'mouse_id = 0').delete()
The contents of the following tables are about to be deleted:
dj_tutorial.mouse (1 tuples)
Proceed? [yes, No]: no


## What’s next?¶

In this section, we learned how to fetch data from the table using the fetch method. We also met our first query operation, & (restriction) and learned how it can be used to narrow down your query results and fetch them. Finally we learned how to delete table entries using delete` method, and also learned how to delete only specific entires by using restriction on the table. As you progress through the tutorials and create more tables, you will learn additional operations and how to combine them into more powerful yet intuitive queries.

In the next section, we will move forward in our data pipeline creation by creating and linking additional tables together.