Using sqlite on android studio, I have two related tables "A" and "B", that both have a column "id". When I make a join on Android Studio, and try to get the value from "id" I get "B.id" whether I put
cursor.getInt(cursor.getColumnIndex("A.id")); // or
cursor.getInt(cursor.getColumnIndex("id"));
I'm considering changing the name of the columns so I can distinguish them easily, but I've seen that sqlite doesn't have any simple thing like "Alter Table "A" rename column", but I would have to duplicate the table with the correct name and deleting the old one so I'm trying to avoid this solution.
Also a solution I thought is to duplicate the column in the query putting something like "Select A.id, * from...", but is something I'd rather avoid.
Any idea why the code I posted might not work? Thanks
Define an alias on the columns, optionally selecting all other columns with *. These are not mutually exclusive, so the following will work:
SELECT a.id AS a_id, b.id AS b_id, * FROM ...
It will return the aliased columns along with all others, even if the data is redundant:
Columns: a_id, b_id, id, ..., id, ...
Frankly, you can't avoid all solutions. The two solutions that you already listed are about all you have to choose from. Sqlite does not automatically prefix table names to the column names, so there are really no other options.
With a Cursor you can (albeit it not recommended) use specific offsets (what the getColumnIndex method does is return the offset, the get???? methods all take on offset).
So say the query was SELECT * FROM A JOIN B ON A.id = B.maptoAid and
Table A has columns:-
id (offset 0) and
name (offset 1)
and Table B has columns:-
id and (offset 2)
maptoAid (offset 3) and
blah (offset 4)
Then (for the above only, the offsets have to be determined)
to get A.id use cursor.getInt(0);
to get B.id use cursor.getInt(2);
Note The use of offsets is discouraged far better to use column aliases (AS) along with the Cursor getColumnIndex method.
offsets are not tolerant of changes to the query, that is change the number or order of columns and offsets may have to be re-calculated.
names, via the getColumnIndex method (unless duplicating the same name) are tolerant of such changes
Such as
SELECT a.id AS aid, b.id AS bid .... other columns (aliased or not) .... FROM A JOIN B ON A.id = B.maptoAid
Or easier to code bit with greater overheads
SELECT *, A.id AS aid, B.id FROM A JOIN B ON A.id = B.maptoAid
and then use :-
cursor.getInt(cursor.getColumnIndex("aid"));
cursor.getInt(cursor.getColumnIndex("bid"));
I get "B.id" whether I put ...
and
Any idea why the code I posted might not work?
The reason why you get B.id is that the getColumnindex method doesn't finish looping when it finds the column BUT continues the loop, thus returning the last if there are more than one columns with the same name.
Also note (unless fixed) that the Cursor getColumnIndex method is also case sensitive. So cursor.getInt(cursor.getColumnIndex("Aid")) would return -1.
Related
I need get the total SUM for each rows in my query, but I don't want go twice in the table.
I tried do this:
SELECT id, value, SUM(value) as total FROM product
But my result was this:
id value total
3 30 60
If I do the bellow query I get my wanted result, but I need go twice in the table:
SELECT id, value, (SELECT SUM(value) FROM product) as total FROM product
Or if I use 'WITH' clause, but this is not supported before Android 5:
WITH data AS (SELECT id, value FROM product)
SELECT id, value, (SELECT SUM(value) FROM data) as total FROM data
Wanted result:
id value total
1 10 60
2 20 60
3 30 60
Thank you!
It's not possible using your SQLite version. You'll have to use two selects.
Basically you have to use a subquery.
However, perhaps you may be less concerned about the 2nd table as I believe that the Query Planner will determine that it only needs to calculate the sum once and does away with the need for a variable as it stores the value in cache.
I believe that the results of using EXPLAIN QUERY PLAN your_query shows this. i.e. using
EXPLAIN QUERY PLAN SELECT id, value, (SELECT sum(value) FROM products) AS total FROM products;
results in :-
This being explained as (see bolded statements) :-
1.3. Subqueries
In all the examples above, the first column (column "selectid") is
always set to 0. If a query contains sub-selects, either as part of
the FROM clause or as part of SQL expressions, then the output of
EXPLAIN QUERY PLAN also includes a report for each sub-select. Each
sub-select is assigned a distinct, non-zero "selectid" value. The
top-level SELECT statement is always assigned the selectid value 0.
For example:
sqlite> EXPLAIN QUERY PLAN SELECT (SELECT b FROM t1 WHERE a=0), (SELECT a FROM t1 WHERE b=t2.c) FROM t2;
0|0|0|SCAN TABLE t2
0|0|0|EXECUTE SCALAR SUBQUERY 1
1|0|0|SEARCH TABLE t1 USING COVERING INDEX i2 (a=?)
0|0|0|EXECUTE CORRELATED SCALAR SUBQUERY 2
2|0|0|SEARCH TABLE t1 USING INDEX i3 (b=?)
The example above contains a pair of scalar subqueries assigned
selectid values 1 and 2. As well as a SCAN record, there are also 2
"EXECUTE" records associated with the top level subquery (selectid 0),
indicating that subqueries 1 and 2 are executed by the top level query
in a scalar context. The CORRELATED qualifier present in the EXECUTE
record associated with scalar subquery 2 indicates that the query must
be run separately for each row visited by the top level query. Its
absence in the record associated with subquery 1 means that the
subquery is only run once and the result cached. In other words,
subquery 2 may be more performance critical, as it may be run many
times whereas subquery 1 is only ever run once.
Unless the flattening optimization is applied, if a subquery appears
in the FROM clause of a SELECT statement, SQLite executes the subquery
and stores the results in a temporary table. It then uses the contents
of the temporary table in place of the subquery to execute the parent
query. This is shown in the output of EXPLAIN QUERY PLAN by
substituting a "SCAN SUBQUERY" record for the "SCAN TABLE" record that
normally appears for each element in the FROM clause. For example:
sqlite> EXPLAIN QUERY PLAN SELECT count(*) FROM (SELECT max(b) AS x FROM t1 GROUP BY a) GROUP BY x;
1|0|0|SCAN TABLE t1 USING COVERING INDEX i2
0|0|0|SCAN SUBQUERY 1
0|0|0|USE TEMP B-TREE FOR GROUP BY
If the flattening optimization is used on a subquery in the FROM
clause of a SELECT statement, then the output of EXPLAIN QUERY PLAN
reflects this. For example, in the following there is no "SCAN
SUBQUERY" record even though there is a subquery in the FROM clause of
the top level SELECT. Instead, since the flattening optimization does
apply in this case, the EXPLAIN QUERY PLAN report shows that the top
level query is implemented using a nested loop join of tables t1 and
t2.
sqlite> EXPLAIN QUERY PLAN SELECT * FROM (SELECT * FROM t2 WHERE c=1), t1;
0|0|0|SEARCH TABLE t2 USING INDEX i4 (c=?)
0|1|1|SCAN TABLE t1
EXPLAIN QUERY PLAN
End Note
Perhaps of relevance is this statement :-
The best feature of SQL (in all its implementations, not just SQLite)
is that it is a declarative language, not a procedural language. When
programming in SQL you tell the system what you want to compute, not
how to compute it. The task of figuring out the how is delegated to
the query planner subsystem within the SQL database engine.
Query Planning
You may also find this of interest he SQLite Query Optimizer Overview noting that as of release 3.8.0 The Next-Generation Query Planner is utilised.
I'm trying to get the lowest _id from my fts table with this query:
SELECT MIN(_id) FROM fts WHERE tbl_no=2 AND parent_id=6
The result I'm getting is 10. However the smallest _id is 9 and it fits the selection arguments.
If I instead use
SELECT _id FROM fts WHERE tbl_no=2 AND parent_id=6
and select the 1st row, I get the correct result: 9.
Does have something to do with the table being virtual (FTS)? I recently transfered from multiple tables to a single FTS and am experiencing this.
Am I guaranteed to get the results I want with the 2nd query, considering the table never updated and it's sorted by default.
Notes: I am running this on Android (tried rawQuery and query). I have the table in front of me and I know it's correct:
Is _id a numeric or a string?
With string comparison, '10' < '9'.
Try:
SELECT MIN(CAST(_id AS UNSIGNED)) FROM fts WHERE tbl_no=2 AND parent_id=6
To check. I would not use this in production however as it won't be able to use an index.
In FTS tables, all columns store string values, and the string '10' is lexicographically smaller than '9'.
Furthermore, MIN(SomeColumn) is not a full-text search query, and thus is not very efficient.
For a unique integer ID in FTS tables, you should use the internal docid column.
I have 2 tables A and B.
Table A contains names and table B contains selected names.
Now I would like to perform the following query on these tables using greendao, Please let me know if it is possible and if it is not are there any alternatives (maybe a raw query).
select *
from A inner join B
on A.nameid = B.nameid
Also, Table A columns: id, nameid, name
and Table B columns: id, nameid, name, rating
I think this might help.
You can use the raw query as a fake join. And you get all you want in the Query object
Query query = ATableDao.queryBuilder().where(
new StringCondition("nameid IN " +
"(SELECT nameid FROM B_Table )").build();
Since "nameid" doesn't seems to be a unique identifier in your sample. I won't suggest to use Relations to solve this issue. If you are try to use Relations, you can find my previous answer here.
Try this:
List<ATableObj> listATableObj = ATableDao.queryRawCreate(", BTable BT
WHERE BT.nameid = T.nameid").list();
If you use greendao this works differntly:
Instead of your query you select rows from table a (or b) and if you need a field of b (or a) you call getB() (or getA()) to get the corresponding row of that table.
If you have rows in table a that have no match in table b and you have rows in table b that have no match in a and you onlly want to select everything that has matches uin both tables, you would have to do a raw query to filter the rows of a (or b).
This is for Android SQLite. I have two queries like this:
select * from table where name='name';
and
select * from table where name!='name' order by name;
I want to create a statement which combines these two queries. I tried union all but I can't do order by one statement and then combine. I tried this:
select * from table where name='name'
union all
select * from table where name!='name' order by name;
All it did is to combine the queries and then order by name. I don't want that. I want to do order by on the second statement first and then combine them.
To put the question differently, here is my data:
Name
a
b
c
d
e
f
g
h
i
j
But I want the output to be:
Name
g
a
b
c
d
e
f
h
i
j
I want to get one row to the top and then order the rest of the rows. Any help is appreciated.
No need to use temporary tables, you need to add an additional column to sort on. Something like this:
select 1, * from table where name='name'
union all
select 2, * from table where name!='name'
order by 1, name;
I don't have a sqlite install right now, but this trick should work. (you may have to add an alias to the first column).
Unless there is some other attribute of the table you can use to provide sorting that allows a join between the two selects as in How to combine two sql queries? then I think you'll have to store the result of the query that should float to the top in a temporary table and then add the sorted results to that table before storing it.
I've never used temporary tables in Android so can't provide an example but as far as I'm aware it's possible.
I'd recommend running the two queries separately and then combining the results in code if that's possible in your situation.
According to the SQLLite docs this cannot be done with a UNION or UNION ALL because those operations must be performed on a simple select, (ones without Order by).
http://www.sqlite.org/lang_select.html
There's probably a very clever way to do this that I don't know, which generally leads me to just do two queries and combine the results in java.
[EDIT] And Jhovanny has the very clever way to do it.
Can't test it right now, but something like this should work:
select t.*, case when name = 'name' then 0 else 1 as o from table t order by o, name;
Then you don't have the two selects nor the union. Assuming you can use a case statement in sqlite on android.
i'm writing an Android app and i've run into a bit of a roadblock involving databases. the way Android handles databases, i cannot refer to names in the result set by the usual 'tablename.colname' method, so this presents a huge issue when any tables in the database contain the same column name. what further complicates the issue, is that any table that is used by a ViewAdapter to display the data to the user (as in my application), must contain a field named "_id" as an autoincrement primary key int. therefore, some tables MUST have identical column names. however, to avoid this, it is possible to use an "AS" clause in a statement to rename the value in question. however, i'm using a rather long statement and i don't know how to limit the columns returned on a JOINed table. what i have is this, and it's completely illegal in android due to the 'tablename.colname' references. i actually added the table names in to make the statement more readable, but i can't use them:
SELECT call._id AS android_call_id,
call.phone,
call.time,
call.duration
call.duration_billed
call.pending
call.call_id
call.job_id
FROM call
LEFT OUTER JOIN phone ON call.phone_number=phone.phone
LEFT OUTER JOIN job ON job._id=call.job_id
WHERE call.pending=1 ORDER BY job._id
but what i need, is to rename the job._id to something else using an "AS" statement, same as with the 'call._id' field in the first part of the query. how do i achieve this renaming in a JOIN?
edit:
progress so far. i think i've worked out the syntax errors, but i get another runtime error "no such column 'job._id', which may be related to #Tom H. comment
edit 2:
turns out Tom was right, and i adjusted accordingly, but it doesn't work:
SELECT call._id AS android_call_id,
call.phone,
call.time,
call.duration,
call.duration_billed,
call.pending,
call.call_id,
call.job_id,
job._id AS android_job_id,
job.job_name,
job.job_number
FROM call
LEFT OUTER JOIN phone ON call.phone_number=phone.phone
LEFT OUTER JOIN job ON job._id=call.job_id
WHERE call.pending=1 ORDER BY job._id
error:
05-24 16:50:37.561: ERROR/Minutemaid - Service(7705): oops: ambiguous column name: call._id: , while compiling: SELECT call._id AS android_call_id,call.phone_number,call.time,call.duration,call.duration_billed,call.pending,call.call_id,call.job_id,job._id AS android_job_id,job.job_name,job.job_number FROM call LEFT OUTER JOIN phone ON call.phone_number=phone.phone LEFT OUTER JOIN call ON call.job_id=job._id WHERE call.pending=1 ORDER BY job._id
Can't you simply use AS to alias all of the tablename.columnname references to unique names in the result set?
You can simply create a VIEW that restricts columns selectable in a table and assigns another name to them.
You can try massaging the table names before you join them by using sub-queries with AS in the FROM clause. For example:
select c_phone, c_id, p_id
from (select id as c_id, phone as c_phone, phone_number as c_phone_number, ... from call) as c
left outer join (select id as p_id, phone as p_phone, ... ) as p
on c_phone_number = p_phone
...
If the limitation is just that you can't use table names to distinguish between columns but can use correlation names then simpler is:
select c.id, c.phone, p.id as "p_id" from ... call c join phone p