I'm migrating mySQL to DynamoDB. In mySQL, I have
SELECT * FROM mytable WHERE userId IN myList
How can I achieve it in DynamoDB?
Thanks
You can use a filter expression on a Scan operation to achieve the same result as the SQL query above. For example, if you use the Document SDK in Java, you could write the following:
final Table table = new Table(AmazonDynamoDBClient.builder().withRegion(Regions.US_EAST_1).build(), "mytable");
//convert the Iterable<Item> returned by table.scan() to a stream of Items
StreamSupport.stream(
// list however many items you need to test after IN
table.scan(new ScanSpec().withFilterExpression("userId IN :u1, :u2")
//define the values of the set of usernames you are testing in the list avove
.withValueMap(ImmutableMap.of(":u1", "robert", ":u2", "daniel"))).spliterator(), false)
//do useful stuff with the result set
.map(Item::toJSONPretty)
.forEach(System.out::println);
Related
I'm having some strange feeling abour sqlite3 parameters that I would like to expose to you.
This is my query and the fail message :
#query
'SELECT id FROM ? WHERE key = ? AND (userid = '0' OR userid = ?) ORDER BY userid DESC LIMIT 1;'
#error message, fails when calling sqlite3_prepare()
error: 'near "?": syntax error'
In my code it looks like:
// Query is a helper class, at creation it does an sqlite3_preprare()
Query q("SELECT id FROM ? WHERE key = ? AND (userid = 0 OR userid = ?) ORDER BY userid DESC LIMIT 1;");
// bind arguments
q.bindString(1, _db_name.c_str() ); // class member, the table name
q.bindString(2, key.c_str()); // function argument (std::string)
q.bindInt (3, currentID); // function argument (int)
q.execute();
I have the feeling that I can't use sqlite parameters for the table name, but I can't find the confirmation in the Sqlite3 C API.
Do you know what's wrong with my query?
Do I have to pre-process my SQL statement to include the table name before preparing the query?
Ooookay, should have looked more thoroughly on SO.
Answers:
- SQLite Parameters - Not allowing tablename as parameter
- Variable table name in sqlite
They are meant for Python, but I guess the same applies for C++.
tl;dr:
You can't pass the table name as a parameter.
If anyone have a link in the SQLite documentation where I have the confirmation of this, I'll gladly accept the answer.
I know this is super old already but since your query is just a string you can always append the table name like this in C++:
std::string queryString = "SELECT id FROM " + std::string(_db_name);
or in objective-C:
[#"SELECT id FROM " stringByAppendingString:_db_name];
In my db-driven app I need to perform insert into queries in which the value for one or more field comes from a subquery.
The insert into statement may look like the following example:
INSERT INTO MyTable (field_1, field_2)
VALUES('value for field 1', (SELECT field_x FROM AnotherTable WHERE ...))
At present I am doing it manually building the query:
String MyQuery = "INSERT INTO mytable (field_1, field_2)
VALUES('value for field 1', (SELECT field_x FROM AnotherTable WHERE ...))"; // Of course my query is far more complex and is built in several steps but the concept is safe, I end up with a SQL String
SQLiteDatabase= db = getWritableDatabase();
db.execSQL(MyQuery); // And it works flawlessy as it was a Swiss Clock
What i would like to do instead is:
SQLiteDatabase db = getWritableDatabase();
ContentValues values = new ContentValues();
values.put("field_1", "value for field 1");
values.put("field_2", ThisIsAQuery("(SELECT field_x FROM AnotherTable WHERE ...)"));
db.insert("MyTable", null, values);
db.close();
Where the fake method ThisIsAQuery(...) is the missing part, something that should tell the query builder that "SELECT.." is not a value but a query that should be embedded in the insert statement.
Is there a way to achieve this?
The whole point of the ContentValues container is to be able to safely use strings without interpreting them as SQL commands.
It is not possible to use subqueries with insert(). The only way to get a value from another table is by executing a separate query; in this case, ThisIsAQuery() would be stringForQuery() or longForQuery().
i am bit confused by the nature and working of query , I tried to access database which contains each name more than once having same EMPid so when i accessed it in my DROP DOWN LIST then same repetition was in there too so i tried to remove repetition by putting DISTINCT in query but that didn't work but later i modified it another way and that worked but WHY THAT WORKED, I DON'T UNDERSTAND ?
QUERY THAT DIDN'T WORK
var names = (from n in DataContext.EmployeeAtds select n).Distinct();
QUERY THAT WORKED of which i don't know how ?
var names = (from n in DataContext.EmployeeAtds select new {n.EmplID, n.EmplName}).Distinct();
why 2nd worked exactly like i wanted (picking each name 1 time)
i'm using mvc 3 and linq to sql and i am newbie.
Both queries are different. I am explaining you both query in SQL that will help you in understanding both queries.
Your first query is:
var names = (from n in DataContext.EmployeeAtds select n).Distinct();
SQL:-
SELECT DISTINCT [t0].[EmplID], [t0].[EmplName], [t0].[Dept]
FROM [EmployeeAtd] AS [t0]
Your second query is:
(from n in EmployeeAtds select new {n.EmplID, n.EmplName}).Distinct()
SQL:-
SELECT DISTINCT [t0].[EmplID], [t0].[EmplName] FROM [EmployeeAtd] AS
[t0]
Now you can see SQL query for both queries. First query is showing that you are implementing Distinct on all columns of table but in second query you are implementing distinct only on required columns so it is giving you desired result.
As per Scott Allen's Explanation
var names = (from n in DataContext.EmployeeAtds select n).Distinct();
The docs for Distinct are clear – the method uses the default equality comparer to test for equality, and the default comparer sees 4 distinct object references. One way to get around this would be to use the overloaded version of Distinct that accepts a custom IEqualityComparer.
var names = (from n in DataContext.EmployeeAtds select new {n.EmplID, n.EmplName}).Distinct();
Turns out the C# compiler overrides Equals and GetHashCode for anonymous types. The implementation of the two overridden methods uses all the public properties on the type to compute an object's hash code and test for equality. If two objects of the same anonymous type have all the same values for their properties – the objects are equal. This is a safe strategy since anonymously typed objects are essentially immutable (all the properties are read-only).
Try this:
var names = DataContext.EmployeeAtds.Select(x => x.EmplName).Distinct().ToList();
Update:
var names = DataContext.EmployeeAtds
.GroupBy(x => x.EmplID)
.Select(g => new { EmplID = g.Key, EmplName = g.FirstOrDefault().EmplName })
.ToList();
I need to get column names and their tables in a SQLite database. What I need is a resultset with 2 columns: table_name | column_name.
In MySQL, I'm able to get this information with a SQL query on database INFORMATION_SCHEMA. However the SQLite offers table sqlite_master:
sqlite> create table students (id INTEGER, name TEXT);
sqlite> select * from sqlite_master;
table|students|students|2|CREATE TABLE students (id INTEGER, name TEXT)
which results a DDL construction query (CREATE TABLE) which is not helpful for me and I need to parse this to get relevant information.
I need to get list of tables and join them with columns or just get columns along with table name column. So PRAGMA table_info(TABLENAME) is not working for me since I don't have table name. I want to get all column metadata in the database.
Is there a better way to get that information as a result set by querying database?
You've basically named the solution in your question.
To get a list of tables (and views), query sqlite_master as in
SELECT name, sql FROM sqlite_master
WHERE type='table'
ORDER BY name;
(see the SQLite FAQ)
To get information about the columns in a specific table, use PRAGMA table_info(table-name); as explained in the SQLite PRAGMA documentation.
I don't know of any way to get tablename|columnname returned as the result of a single query. I don't believe SQLite supports this. Your best bet is probably to use the two methods together to return the information you're looking for - first get the list of tables using sqlite_master, then loop through them to get their columns using PRAGMA table_info().
Recent versions of SQLite allow you to select against PRAGMA results now, which makes this easy:
SELECT
m.name as table_name,
p.name as column_name
FROM
sqlite_master AS m
JOIN
pragma_table_info(m.name) AS p
ORDER BY
m.name,
p.cid
where p.cid holds the column order of the CREATE TABLE statement, zero-indexed.
David Garoutte answered this here, but this SQL should execute faster, and columns are ordered by the schema, not alphabetically.
Note that table_info also contains
type (the datatype, like integer or text),
notnull (1 if the column has a NOT NULL constraint)
dflt_value (NULL if no default value)
pk (1 if the column is the table's primary key, else 0)
RTFM: https://www.sqlite.org/pragma.html#pragma_table_info
There are ".tables" and ".schema [table_name]" commands which give kind of a separated version to the result you get from "select * from sqlite_master;"
There is also "pragma table_info([table_name]);" command to get a better result for parsing instead of a construction query:
sqlite> .tables
students
sqlite> .schema students
create table students(id INTEGER, name TEXT);
sqlite> pragma table_info(students);
0|id|INTEGER|0||0
1|name|TEXT|0||0
Hope, it helps to some extent...
Another useful trick is to first get all the table names from sqlite_master.
Then for each one, fire off a query "select * from t where 1 = 0". If you analyze the structure of the resulting query - depends on what language/api you're calling it from - you get a rich structure describing the columns.
In python
c = ...db.cursor()
c.execute("select * from t where 1=0");
c.fetchall();
print c.description;
Juraj
PS. I'm in the habit of using 'where 1=0' because the record limiting syntax seems to vary from db to db. Furthermore, a good database will optimize out this always-false clause.
The same effect, in SQLite, is achieved with 'limit 0'.
FYI, if you're using .Net you can use the DbConnection.GetSchema method to retrieve information that usually is in INFORMATION_SCHEMA. If you have an abstraction layer you can have the same code for all types of databases (NOTE that MySQL seems to swich the 1st 2 arguments of the restrictions array).
Try this sqlite table schema parser, I implemented the sqlite table parser for parsing the table definitions in PHP.
It returns the full definitions (unique, primary key, type, precision, not null, references, table constraints... etc)
https://github.com/maghead/sqlite-parser
The syntax follows sqlite create table statement syntax: http://www.sqlite.org/lang_createtable.html
This is an old question but because of the number of times it has been viewed we are adding to the question for the simple reason most of the answers tell you how to find the TABLE names in the SQLite Database
WHAT DO YOU DO WHEN THE TABLE NAME IS NOT IN THE DATABASE ?
This is happening to our app because we are creating TABLES programmatically
So the code below will deal with the issue when the TABLE is NOT in or created by the Database Enjoy
public void toPageTwo(View view){
if(etQuizTable.getText().toString().equals("")){
Toast.makeText(getApplicationContext(), "Enter Table Name\n\n"
+" OR"+"\n\nMake Table First", Toast.LENGTH_LONG
).show();
etQuizTable.requestFocus();
return;
}
NEW_TABLE = etQuizTable.getText().toString().trim();
db = dbHelper.getWritableDatabase();
ArrayList<String> arrTblNames = new ArrayList<>();
Cursor c = db.rawQuery("SELECT name FROM sqlite_master WHERE
type='table'", null);
if (c.moveToFirst()) {
while ( !c.isAfterLast() ) {
arrTblNames.add( c.getString( c.getColumnIndex("name")) );
c.moveToNext();
}
}
c.close();
db.close();
boolean matchFound = false;
for(int i=0;i<arrTblNames.size();i++) {
if(arrTblNames.get(i).equals(NEW_TABLE)) {
Intent intent = new Intent(ManageTables.this, TableCreate.class
);
startActivity( intent );
matchFound = true;
}
}
if (!matchFound) {
Toast.makeText(getApplicationContext(), "No Such Table\n\n"
+" OR"+"\n\nMake Table First", Toast.LENGTH_LONG
).show();
etQuizTable.requestFocus();
}
}
I've got two un-related (no FK's defined) tables. The first table contains some tasks for which a user may not have access. I need to find all those tasks - in this case, the joined table would contain nulls. How do I get them?
Here's the setup:
TimeData table
- userID
- taskID
- hours
ApprovedTasks table (the one that should contain nulls)
- taskID
- userID
The SQL query would look like this:
select * from TimeData td
left join ApprovedTasks at
on at.taskID = td.taskID and at.userID = td.userID
where at.taskID is null
Any way to pull that off using a LINQ to Entity query?
TIA
Check out... Disjoint Union in LINQ
This should work...
var approvedTaks = from at in ApprovedTasks.Except(
from at2 in ApprovedTasks
where at2.userID == userId and at2.taskID==taskId
select at2)
where at.userID == userId and at.taskID==taskId
select at;
but sorry don't have the database handy to test it.