My table looks like this,
CREATE TABLE IF NOT EXISTS names (
firstname text,
surname text,
id text,
PRIMARY KEY (firstname, surname)
)
Let's say I want to return a minimum of 10 names.
I do
select * from names where firstname = "something" and surname "something";
But if this only returns 6 people, I then want it to do:
select * from names where firstname = "something" limit 4;
But I want to avoid returning the same row twice.
And possibly do it in one query only.
Is this possible?
You may use SELECT "DISTINCT" feature of CQLSH. you will get unique value for partitions. Also please refer below documentations for more understanding:-
https://docs.datastax.com/en/dse/5.1/cql/cql/cql_reference/cql_commands/cqlSelect.html
You can rely on the paging implemented by driver(s), for example, in Java.
In your case, you can perform query, and use .setFetchSize when performing specific query to some value you need - in this case, driver will read approximately specified number (or less) as a first page, and if you'll need more, then you can proceed iterating over results, and driver will fetch next page, until either you stop, or there will be no more data.
But be very careful with too low values of the page - if you have a lot of data in partition, then driver will need to go to Cassandra very often, and this will impact performance.
P.S. you can't have 10 records for query where firstname = "something" and surname = "something", because both columns comprise full primary key, and there could be only one record for given primary key. You may use something like where firstname = "something" and surname >= "something" to fetch data starting with given surname.
Related
I need to build one MSSQL query that selects one row that is the best match.
Ideally, we have a match on street, zip code and house number.
Only if that does not deliver any results, a match on just street and zip code is sufficient
I have this query so far:
SELECT TOP 1 * FROM realestates
WHERE
(Address_Street = '[Street]'
AND Address_ZipCode = '1200'
AND Address_Number = '160')
OR
(Address_Street = '[Street]'
AND Address_ZipCode = '1200')
MSSQL currently gives me the result where the Address_Number is NOT 160, so it seems like the 2nd clause (where only street and zipcode have to match) is taking precedence over the 1st. If I switch around the two OR clauses, same result :)
How could I prioritize the first OR clause, so that MSSQL stops looking for other results if we found a match where the three fields are present?
The problem here isn't the WHERE (though it is a "problem"), it's the lack of an ORDER BY. You have a TOP (1), but you have nothing that tells the data engine which row is the "top" row, so an arbitrary row is returned. You need to provide logic, in the ORDER BY to tell the data engine which is the "first" row. With the rudimentary logic you have in your question, this would like be:
SELECT TOP (1)
{Explicit Column List}
realestates
WHERE Address_Street = '[Street]'
AND Address_ZipCode = '1200'
ORDER BY CASE Address_Number WHEN '160' THEN 1 ELSE 2 END;
You can't prioritize anything in the WHERE clause. It always results in ALL the matching rows. What you can do is use TOP or FETCH to limit how many results you will see.
However, in order for this to be effective, you MUST have an ORDER BY clause. SQL tables are unordered sets by definition. This means without an ORDER BY clause the database is free to return rows in any order it finds convenient. Mostly this will be the order of the primary key, but there are plenty of things that can change this.
I usually use jsonb field store array data.
for example, I want to store customer's barcode info, I will create a table like this:
create table customers(fcustomerid bigint, fcodes jsonb);
One customer has one row, all barcode info stored in its fcodes field, just like below:
[
{
"barcode":"000000001",
"codeid":1,
"product":"Coca Cola",
"createdate":"2021-01-19",
"lottorry":true,
"lottdate":"2021-01-20",
"bonus":50
},
{
"barcode":"000000002",
"codeid":2,
"product":"Coca Cola",
"createdate":"2021-01-19",
"lottorry":false,
"lottdate":"",
"bonus":0
}
...
{
"barcode":"000500000",
"codeid":500000,
"product":"Pepsi Cola",
"createdate":"2021-01-19",
"lottorry":false,
"lottdate":"",
"bonus":0
}
]
The jsonb array maybe store millions of barcode's objects with the same structure. Perhaps this is not a good idea, but you konw when I have thousands of customer, I can store all the data in one table, one customer has one row in this table, all its data store in one field, it looks very tersely and easy to manage.
For this kind of application scenarios, how to efficiently to insert or modify or query the data?
I can use jsonb_insert to insert one object, just like:
update customers
set fcodes=jsonb_insert(fcodes,'{-1}','{...}'::jsonb)
where fcustomerid=999;
When I want modify some object, I found it is a little difficulty, I should know the index of object first, if I use the incremental key codeid as the array index, things looks easilly. I can use jsonb_modify,Just like below:
update customers
set fcodes=jsonb_set(fcodes,concat('{',(mycodeid-1)::text,',lottery}'),'true'::jsonb)
where fcustomerid=999;
But if I want to query the objects in the jsonb array with createdate or bonus or lottorry or product, I should use jsonpath operator. just like:
select jsonb_path_query_array(fcodes,'$ ? (product=="Pepsi Cola")'
from customer
where fcustomerid=999;
or like:
select jsonb_path_query_array(fcodes,'$ ? (lottdate.datetime()>="2021-01-01".datetime() && lottdate.datetime()<="2021-01-31".datetime())'
from customer
where fcustomerid=999;
Thie jsonb index looks useful, But it looks useful between different row, and my operation mostly works in one row's one jsonb field.
I am very worrying about the efficiency, for millions of objects stored in one row's one jsonb field, is this a good idea? And how to improve the efficiency in this scenarios? Especially for the query.
You are right to worry. With a huge JSON like that, you will never get good performance.
Your data don't need JSON at all. Create a table that stores a single barcode and has a foreign key reference to customers. Then everything will be simple and efficient.
Using JSON in the database is almost always the wrong choice, judging from the questions in this forum.
I'm using DSE solr to index a cassandra table that contains a collection of UDTs. I want to be able to sort search results based on a value inside those UDTs.
Given a simplistic example table...
create type test_score (
test_name text,
percentile double,
score int,
description text
);
create table students (
id int,
name text,
test_scores set<frozen<test_score>>,
...
);
... and assuming I'm auto-generating the solr schema via dsetool, I want to be able to write a solr query that finds students who have taken a test (by a specific test_name), and sort them by that test's score (or percentile, or whatever).
Unfortunately you can't sort by UDT fields.
However, I'm not sure what the value of a UDT is here. Perhaps I don't know enough about your use case. Another issue I see is that each partition key is a student id, so you can only store one test result per student. A better approach might be to use test id as a clustering column so you can store all the test results for a student in a single partition. Something like this:
CREATE TABLE students (
id int,
student_name text,
test_name text,
score int,
percentile double,
description text,
PRIMARY KEY (id, student_name, test_name)
);
Student name is kind of redundant (it should be the same for every row in each partition), but it doesn't have to be a clustering column.
Then you can sort on any field like so:
SELECT * FROM students WHERE solr_query='{"q":"test_name:Biology", "sort":"percentile desc"}' LIMIT 10;
I've used the JSON syntax described here: https://docs.datastax.com/en/datastax_enterprise/4.8/datastax_enterprise/srch/srchJSON.html
Ok so basically you want to do a JOIN between table test_score and students right ?
According to the official doc: http://docs.datastax.com/en/datastax_enterprise/4.8/datastax_enterprise/srch/srchQueryJoin.html
Joining Solr core is possible only if the 2 tables share the same partition key, which is not the case in your example ...
Ok so I have an application that uses GAE and consequently the datastore.
Say I have multiple companies A, B and C and I have within each company Employees X,Y and Z. The relationship between a company and employee will be OneToMany, with the company being the owner. This results in the Company Key being of the form
long id = 4504699138998272; // Random Example
Key CompanyKey = KeyFactory.createKey(Company.class.getSimpleName(), id);
and the employee key would be of the form
long id2 = 5630599045840896;
Key EmployeeKey = KeyFactory.createKey(CompanyKey,Employee.class.getSimpleName(),id2);
all fine and well and there is no problem, until in the front end, during jsp representation. Sometimes I would need to generate a report, or open an Employees profile, in which case the div containing his information would get an id as follows
<div class="employeeInfo" id="<%=employee.getKey().getId()%>" > .....</div>
and this div has an onclick / submit event, that will ajax the new modifications to the employee profile to servelet, at which point I have to specify the primary key of the employee, (which I thought I could easily get from the div id), but it didnt work server side.
The problem is I know the Employees String portion of the Key and the long portion, but not the Parent key. To save time I tried this and it didnt work
Key key = KeyFactory.creatKey(Employee.class.getSimpleName(); id);
Employee X = em.find(Employee.class,key);
X is always returned null.
I would really appreciate any idea of how to find or "query" Entities by keys without knowing their parents key (as I would hate having to re-adjust Entity classes)
Thanks alot !!
An Entity key and its parents cannot be separated. It's called ancestor path, a chain composed of entity kinds and ids.
So, in your example ancestor paths will look like this:
CompanyKey: ("Company", 4504699138998272)
EmployeeKey: ("Company", 4504699138998272, "Employee", 5630599045840896)
A key composed only of ("Employee", 5630599045840896) is a completely different one comparing to the EmployeeKey even though both keys end with the same values. Think of concatenating elements into a single "string" and comparing final values, they will never match.
One thing you can do is use encoded keys instead of their id values:
String encodedKey = KeyFactory.keyToString(EmployeeKey);
Key decodedKey = KeyFactory.stringToKey(encodedKey);
decodedKey.equals(EmployeeKey); // true
More about Ancestor Paths:
https://developers.google.com/appengine/docs/java/datastore/entities#Java_Ancestor_paths
KeyFactory Java doc:
https://developers.google.com/appengine/docs/java/javadoc/com/google/appengine/api/datastore/KeyFactory#keyToString(com.google.appengine.api.datastore.Key)
I'm developping a web application using google appengine and django, but I think my problem is more general.
The users have the possibility to create tables, look: tables are not represented as TABLES in the database. I give you an example:
First form:
Name of the the table: __________
First column name: __________
Second column name: _________
...
The number of columns is not fixed, but there is a maximum (100 for example). The type in every columns is the same.
Second form (after choosing a particular table the user can fill the table):
column_name1: _____________
column_name2: _____________
....
I'm using this solution, but it's wrong:
class Table(db.Model):
name = db.StringProperty(required = True)
class Column(db.Model):
name = db.StringProperty(required = True)
number = db.IntegerProperty()
table = db.ReferenceProperty(table, collection_name="columns")
class Value(db.Model):
time = db.TimeProperty()
column = db.ReferenceProperty(Column, collection_name="values")
when I want to list a table I take its columns and from every columns I take their values:
data = []
for column in data.columns:
column_data = []
for value in column.values:
column_data.append(value.time)
data.append(column_data)
data = zip(*data)
I think that the problem is the order of the values, because it is not true that the order for one column is the same for the others. I'm waiting for this bug (but until now I never seen it):
Table as I want: as I will got:
a z c a e c
d e f d h f
g h i g z i
Better solutions? Maybe using ListProperty?
Here's a data model that might do the trick for you:
class Table(db.Model):
name = db.StringProperty(required=True)
owner = db.UserProperty()
column_names = db.StringListProperty()
class Row(db.Model):
values = db.ListProperty(yourtype)
table = db.ReferenceProperty(Table, collection_name='rows')
My reasoning:
You don't really need a separate entity to store column names. Since all columns are of the same data type, you only need to store the name, and the fact that they are stored in a list gives you an implicit order number.
By storing the values in a list in the Row entity, you can use an index into the column_names property to find the matching value in the values property.
By storing all of the values for a row together in a single entity, there is no possibility of values appearing out of their correct order.
Caveat emptor:
This model will not work well if the table can have columns added to it after it has been populated with data. To make that possible, every time that a column is added, every existing row belonging to that table would have to have a value appended to its values list. If it were possible to efficiently store dictionaries in the datastore, this would not be a problem, but list can really only be appended to.
Alternatively, you could use Expando...
Another possibility is that you could define the Row model as an Expando, which allows you to dynamically create properties on an entity. You could set column values only for the columns that have values in them, and that you could also add columns to the table after it has data in it and not break anything:
class Row(db.Expando):
table = db.ReferenceProperty(Table, collection_name='rows')
#staticmethod
def __name_for_column_index(index):
return "column_%d" % index
def __getitem__(self, key):
# Allows one to get at the columns of Row entities with
# subscript syntax:
# first_row = Row.get()
# col1 = first_row[1]
# col12 = first_row[12]
value = None
try:
value = self.__dict__[Row.__name_for_column_index]
catch KeyError:
# The given column is not defined for this Row
pass
return value
def __setitem__(self, key, value):
# Allows one to set the columns of Row entities with
# subscript syntax:
# first_row = Row.get()
# first_row[5] = "New values for column 5"
self.__dict__[Row.__name_for_column_index] = value
# In order to allow efficient multiple column changes,
# the put() can go somewhere else.
self.put()
Why don't you add an IntegerProperty to Value for rowNumber and increment it every time you add a new row of values and then you can reconstruct the table by sorting by rowNumber.
You're going to make life very hard for yourself unless your user's 'tables' are actually stored as real tables in a relational database. Find some way of actually creating tables and use the power of an RDBMS, or you're reinventing a very complex and sophisticated wheel.
This is the conceptual idea I would use:
I would create two classes for the data-store:
table this would serve as a
dictionary, storing the structure of
the pseudo-tables your app would
create. it would have two fields :
table_name, column_name,
column_order . where column_order
would give the position of the
column within the table
data
this would store the actual data in
the pseudo-tables. it would have
four fields : row_id, table_name,
column_name , column_data. row_id
would be the same for data
pertaining to the same row and would
be unique for data across the
various pseudo-tables.
Put the data in a LongBlob.
The power of a database is to be able to search and organise data so that you are able to get only the part you want for performances and simplicity issues : you don't want the whole database, you just want a part of it and want it fast. But from what I understand, when you retrieve a user's data, you retrieve it all and display it. So you don't need to sotre the data in a normal "database" way.
What I would suggest is to simply format and store the whole data from a single user in a single column with a suitable type (LongBlob for example). The format would be an object with a list of columns and rows of type. And you define the object in whatever language you use to communicate with the database.
The columns in your (real) database would be : User int, TableNo int, Table Longblob.
If user8 has 3 tables, you will have the following rows :
8, 1, objectcontaintingtable1;
8, 2, objectcontaintingtable2;
8, 3, objectcontaintingtable3;