I have the following schema (SQLite) for capturing messages from messengers:
create table if not exists spot_message
(
id unsigned big int primary key not null,
messenger_id text not null,
messenger_name text not null,
message_type text not null,
timestamp text not null,
latitude real not null,
longitude real not null
);
I use the following self-join in order to find the latest message from every sender:
select t1.*
from spot_message t1
join (select messenger_id, max(timestamp) timestamp
from spot_message
group by messenger_id) t2 on t1.messenger_id = t2.messenger_id and t1.timestamp = t2.timestamp;
It is not clear to me how to represent this in jOOQ.
I currently have:
DSL.using(c.get())
.select(asterisk())
.from(table("spot_message").as("t1"))
.join(select(field("messenger_id"), max(field("timestamp"))).from(table("spot_message"))
.groupBy(field("messenger_id")))
.on(field("messenger_id")
.eq(field("messenger_id"))
.and(field("timestamp")
.eq(field("timestamp"))));
But it is not clear how I express the "as" for the table name of the joined table ("t2").
Answering your question
To alias a Select as a derived table, you can either use:
Select.asTable(alias)
DSL.table(select).as(alias)
You're using the plain SQL API for query building, so you can just use any SQL expression in your strings, such as e.g. "t2.messenger_id". This should work:
DSL.using(c.get())
.select(asterisk())
.from(table("spot_message").as("t1"))
.join(select(field("messenger_id"), max(field("timestamp")))
.from(table("spot_message"))
.groupBy(field("messenger_id")).asTable("t2")) // t2 alias here
.on(field("t1.messenger_id") // Qualifications here, and below
.eq(field("t2.messenger_id"))
.and(field("t1.timestamp")
.eq(field("t2.timestamp"))));
However, this would be a bit more readable if you were using code generation, which I recommend for various reasons
SpotMessage t1 = SPOT_MESSAGE.as("t1");
SpotMessage t2 = SPOT_MESSAGE.as("t2");
DSL.using(c.get())
.select(t1.asterisk())
.from(t1)
.join(
select(t2.MESSENGER_ID, max(t2.TIMESTAMP).as(t2.TIMESTAMP))
.from(t2)
.groupBy(t2.MESSENGER_ID).asTable(t2))
.on(t1.MESSENGER_ID.eq(t2.MESSENGER_ID))
.and(t1.TIMESTAMP.eq(t2.TIMESTAMP));
Alternative using window functions
Since you're using SQLite, which supports window functions, why not just use those? You can even use the QUALIFY syntax, which jOOQ can emulate for you if you're using the commercial distributions:
In SQL:
select *
from spot_message
qualify timestamp = max(timestamp) over (partition by messenger_id)
In jOOQ:
ctx.selectFrom(SPOT_MESSAGE)
.qualify(SPOT_MESSAGE.TIMESTAMP.eq(
max(SPOT_MESSAGE.TIMESTAMP).over(partitionBy(SPOT_MESSAGE.MESSENGER_ID))
));
If QUALIFY isn't available to you, you can still emulate it manually by wrapping the query in a derived table:
SELECT *
FROM (
SELECT
spot_message.*,
timestamp = max(timestamp) over (partition by messenger_id) AS is_max
FROM spot_message
) AS t
WHERE is_max
Related
I'm trying to write a java sql query, the simplified table would be table(name,version) with a unique constraint on (name, version).
I'm trying to insert a row into my database with a conditional statement. Meaning that when a entry with the same name exists, it should insert the row with same name and its version increased by 1.
I have tried with the following:
INSERT INTO table(name,version)
VALUES(?, CASE WHEN EXISTS(SELECT name from table where name=?)
THEN (SELECT MAX(version) FROM table WHERE name = ?) +1
ELSE 1 END)
values are sent by user.
My question is, how can I access the 'name' inside the values so I could compare them?
If you want to write this as a single query:
INSERT INTO table (name, version)
SELECT ?, COLAESCE(MAX(t2.version) + 1, 1)
FROM table t2
WHERE t2.name = ?;
That said, this is dangerous. Two threads could execute this query "at the same time" and possibly create the same version number. You can prevent this from happening by adding a unique index/constraint on (name, version).
With the unique index/constraint, one of the updates will fail if there is a conflict.
I see at least two approaches:
1. For each pair of name and version you first query the max version:
SELECT MAX(VERSION) as MAX FROM <table> WHERE NAME = <name>
And then you insert the result + 1 with a corresponding insert query:
INSERT INTO <table>(NAME,VERSION) VALUES (<name>,result+1)
This approach is very straight-forward, easy-to-read and implement, however, not really performant because of so many queries necessary.
You can achieve that with sql alone with sql analytics and window functions, e.g.:
SELECT NAME, ROW_NUMBER() over (partition BY NAME ORDER BY NAME) as VERSION FROM<table>
You can then save the result of this query as a table using CREATE TABLE as SELECT...
(The assumption here is that the first version is 1, if it is not the case, then one could slightly rework the query). This solution would be very performant even for large datasets.
You should get the name before insertion. In your case, if something went wrong then how would you know about it so you get the name before insert query.
Not sure but you try this:
declare int version;
if exists(SELECT name from table where name=?)
then
version = SELECT MAX(version) FROM table WHERE name = ?
version += 1
else
version = 1
end
Regards.
This is actually a bad plan, you might be changing what the user's specified data. That is likely to not be what is desired, maybe they're not trying to create a new version but just unaware that the one wanted already exists. But, you can create a function, which your java calls, not only inserts the requested version or max+1 if the requested version already exists. Moreover it returns the actual values inserted.
-- create table
create table nv( name text
, version integer
, constraint nv_uk unique (name, version)
);
-- function to create version or 1+max if requested exists
create or replace function new_version
( name_in text
, version_in integer
)
returns record
language plpgsql strict
as $$
declare
violated_constraint text;
return_name_version record;
begin
insert into nv(name,version)
values (name_in,version_in)
returning (name, version) into return_name_version;
return return_name_version;
exception
when unique_violation
then
GET STACKED DIAGNOSTICS violated_constraint = CONSTRAINT_NAME;
if violated_constraint like '%nv\_uk%'
then
insert into nv(name,version)
select name_in, 1+max(version)
from nv
where name = name_in
group by name_in
returning (name, version) into return_name_version;
return return_name_version;
end if;
end;
$$;
-- create some data
insert into nv(name,version)
select 'n1', gn
from generate_series( 1,3) gn ;
-- test insert existing
select new_version('n2',1);
select new_version('n1',1);
select *
from nv
order by name, version;
I am retrieving data from database using jdbc. In my code I am using 3-4 tables to get data. But sometimes if table is not present in database my code gives exception. How to handle this situation. I want my code to continue working for other tables even if one table is not present. Please help.
I have wrote a code like this
sql="select * from table"
now Result set and all.
If table is not present in database it give exception that no such table. I want to handle it. In this code I cannot take tables which are already present in advance . I want to check here itself if table is there or not.
Please do not mark it as a duplicate question. The link you shared doesnot give me required answer as in that question they are executing queries in database not through JDBC code
For Sybase ASE the easiest/quickest method would consist of querying the sysobjects table in the database where you expect the (user-defined) table to reside:
select 1 from sysobjects where name = 'table-name' and type = 'U'
if a record is returned => table exists
if no record is returned => table does not exist
How you use the (above) query is up to you ...
return a 0/1-row result set to your client
assign a value to a #variable
place in a if [not] exists(...) construct
use in a case statement
If you know for a fact that there won't be any other object types (eg, proc, trigger, view, UDF) in the database with the name in question then you could also use the object_id() function, eg:
select object_id('table-name')
if you receive a number => the object exists
if you receive a NULL => the object does not exist
While object_id() will obtain an object's id from the sysobjects table, it does not check for the object type, eg, the (above) query will return a number if there's a stored proc named 'table-name'.
As with the select/sysobjects query, how you use the function call in your code is up to you (eg, result set, populate #variable, if [not] exists() construct, case statement).
So, addressing the additional details provided in the comments ...
Assuming you're submitting a single batch that needs to determine table existence prior to running the desired query(s):
-- if table exists, run query(s); obviously if table does not exist then query(s) is not run
if exists(select 1 from sysobjects where name = 'table-name' and type = 'U')
begin
execute("select * from table-name")
end
execute() is required to keep the optimizer from generating an error that the table does not exist, ie, the query is not parsed/compiled unless the execute() is actually invoked
If your application can be written to use multiple batches, something like the following should also work:
# application specific code; I don't work with java but the gist of the operation would be ...
run-query-in-db("select 1 from sysobjects where name = 'table-name' and type = 'U'")
if-query-returns-a-row
then
run-query-in-db("select * from table-name")
fi
This is the way of checking if the table exists and drop it:
IF EXISTS (
SELECT 1
FROM sysobjects
WHERE name = 'a_table'
AND type = 'U'
)
DROP TABLE a_table
GO
And this is how to check if a table exists and create it.
IF NOT EXISTS (
SELECT 1
FROM sysobjects
WHERE name = 'a_table'
AND type = 'U'
)
EXECUTE("CREATE TABLE a_table (
col1 int not null,
col2 int null
)")
GO
(They are different because in table-drop a temporary table gets created, so if you try to create a new one you will get an exception that it already exists)
Before running the query which has some risk in table not existing, run the following sql query and check if the number of results is >= 1. if it is >= 1 then you are safe to execute the normal query. otherwise, do something to handle this situation.
SELECT count(*)
FROM information_schema.TABLES
WHERE (TABLE_SCHEMA = 'your_db_name') AND (TABLE_NAME = 'name_of_table')
I am no expert in Sybase but take a look at this,
exec sp_tables '%', '%', 'master', "'TABLE'"
Sybase Admin
I'm confusing with implementation of CRUD methods for DAODatabase (for Oracle 11 xe).
The problem is that the "U"-method (update) in case of storing in generally to a Map collection inserts a new element or renews it (key-value data like ID:AbstractBusinessObject) in a Map collection. And you don't care about it, when you write something like myHashMap.add(element). This method (update) is widely used in project's business logic.
Obviously, in case of using Oracle I must care about both inserting and renewing of existing elements. But I'm stucked to choose the way how to implement it:
There is no intrinsic function for so-called UPSERT in Oracle (at least in xe11g r2 version). However, I can emulate necessary function by SQL-query like this:
INSERT INTO mytable (id1, t1)
SELECT 11, 'x1' FROM DUAL
WHERE NOT EXISTS (SELECT id1 FROM mytble WHERE id1 = 11);
UPDATE mytable SET t1 = 'x1' WHERE id1 = 11;
(src:http://stackoverflow.com/a/21310345/2938167)
By using this kind of query (first - insert, second - update) I presume that the data mostly will be inserted not updated (at least it will be rather rare). (May it be not optimal for concurrency?).
Ok, it is possible. But at this point I'm confusing to decide:
-- should I write an SQL function (with approriate arguments of course) for this and call it via Java
-- or should I simply handle a serie of queries for preparedStatements and do them via .executeUpdate/.executeQuery? Should I handle the whole UPSERT SQL code for one preparedStatment or split it into several SQL-queries and prepared statements inside one method's body? (I'm using Tomcat's pool of connections and I pass a connection instance via static method getConnection() to each method implementation in DAODatabase) ?
Is there another possibility to solve the UPSERT quest?
The equivalent to your UPSERT statement would seem to be to use MERGE:
MERGE INTO mytable d
USING ( SELECT 11 AS id, 'x1' AS t1 FROM DUAL ) s
ON ( d.id = s.id )
WHEN NOT MATCHED THEN
INSERT ( d.id, d.t1 ) VALUES ( s.id, s.t1 )
WHEN MATCHED THEN
UPDATE SET d.t1 = s.t1;
You could also use (or wrap in a procedure):
DECLARE
p_id MYTABLE.ID%TYPE := 11;
p_t1 MYTABLE.T1%TYPE := 'x1';
BEGIN
UPDATE mytable
SET t1 = p_t1
WHERE id = p_id;
IF SQL%ROWCOUNT = 0 THEN
INSERT INTO mytable ( id, t1 ) VALUES ( p_id, p_t1 );
END IF;
END;
/
However, when you are handling a CRUD request - if you are doing a Create action then it should be represented by an INSERT (and if something already exists then you ought to throw the equivalent of the HTTP status code 400 Bad Request or 409 Conflict, as appropriate) and if you are doing an Update action it should be represented by an UPDATE (and if nothing is there to update then return the equivalent error to 404 Not Found.
So, while MERGE fits your description I don't think it is representative of a RESTful action as you ought to be separating the actions to their appropriate end-points rather than combining then into a joint action.
When I use this statement it does not return results.
Result<Record> result = (Result<Record>) jooq
.select()
.from("Employees")
.where(DSL.cast("FirstName", MySQLDataType.BINARY)
.eq(DSL.cast(firstName, MySQLDataType.BINARY)))
.fetchOne();
I want to select only one Result.
Structure:
--
-- Table structure for table `Employees`
--
CREATE TABLE IF NOT EXISTS `Employees` (
`id` int(11) NOT NULL,
`FirstName` varchar(100) NOT NULL,
`LastName` varchar(150) NOT NULL,
`Age` tinyint(4) NOT NULL DEFAULT '0'
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=6 ;
I want to select is Jack and it exists but it returns null.
Example:
SELECT * FROM Employees WHERE FirstName = Jack;
This:
// Assuming this static import
import static org.jooq.impl.DSL.*;
cast("FirstName", MySQLDataType.BINARY)
Will generate the following SQL
-- With a bind variable:
CAST(? AS BINARY)
-- If you're inlining bind variables:
CAST('FirstName' AS BINARY)
So, this is not referring to your `FirstName` column, but to the 'FirstName' string value. What you really wanted to do is this:
cast(field(name("FirstName")), MySQLDataType.BINARY);
Which will generate
CAST(`FirstName` AS BINARY)
A general note on case sensitivity
If you're using backticks around table / column names in your DDL, you should always be aware of case sensitivity with your object names also in jOOQ. Ideally, you'll use the DSL.name() method as indicated in my answer to create case sensitive names. This also applies to your Employees table, which is added to your SQL statement case insensitively:
from("Employees") // Generates a "plain SQL", case-insensitive table Employees
I recommend you write this instead:
from(name("Employees")) // Generates a case-sensitive table identifier `Employees`
The reason why it works is because MySQL doesn't know case sensitive table names on Windows by default.
Manual references
I suggest reading the jOOQ manual sections about "plain SQL" and "identifiers" to help clarify things:
http://www.jooq.org/doc/latest/manual/sql-building/plain-sql
http://www.jooq.org/doc/latest/manual/sql-building/names
With jooq 3.4 I can't figure out how to do this (with Postgresql):
Query query = dsl.insertInto(TABLE)
.set(TABLE.ID, Sequences.TABLE_ID_SEQ.nextval());
but in a case when I don't know which is the exact table, something like this:
TableImpl<?> tableImpl;
Query query = dsl.insertInto(tableImpl)
.set(tableImpl.getIdentity(), tableImpl.getIdentity().getSequence().nextval());
Is it somehow possible?
I tried this:
dsl.insertInto(tableImpl)
.set(DSL.field("id"),
tableImpl.getSchema().getSequence("table_id_seq").nextval())
This works but I still don't know how to get the sequence name from the TableImpl object.
Is there a solution for this? Or is there a problem with my approach?
In plain SQL I would do this:
insert into table_A (id) VALUES nextval('table_A_id_seq');
insert into table_B (table_A_id, some_val) VALUES (currval('table_A_id_seq'), some_val);
So I need the value or a reference to that id for later use of the id that was generated for the inserted record as default, but I don't want to set any other values.
jOOQ currently doesn't have any means of associating a table with its implicitly used sequence for the identity column. The reason for this is that the sequence is generated when the table is created, but it isn't formally connected to that table.
Usually, you don't have to explicitly set the serial value of a column in a PostgreSQL database. It is generated automatically on insert. In terms of DDL, this means:
CREATE TABLE tablename (
colname SERIAL
);
is equivalent to specifying:
CREATE SEQUENCE tablename_colname_seq;
CREATE TABLE tablename (
colname integer NOT NULL DEFAULT nextval('tablename_colname_seq')
);
ALTER SEQUENCE tablename_colname_seq OWNED BY tablename.colname;
The above is taken from:
http://www.postgresql.org/docs/9.3/static/datatype-numeric.html#DATATYPE-SERIAL
In other words, just leave out the ID values from the INSERT statements.
"Empty" INSERT statements
Note that if you want to create an "empty" INSERT statement, i.e. a statement where you pass no values at all, generating a new column with a generated ID, you can use the DEFAULT VALUES clause.
With SQL
INSERT INTO tablename DEFAULT VALUES
With jOOQ
DSL.using(configuration)
.insertInto(TABLENAME)
.defaultValues()
.execute();
Returning IDs
Note that PostgreSQL has native support for an INSERT .. RETURNING clause, which is also supported by jOOQ:
With SQL
INSERT INTO tablename (...) VALUES (...) RETURNING ID
With jOOQ
DSL.using(configuration)
.insertInto(TABLENAME, ...)
.values(...)
.returning(TABLENAME.ID)
.fetchOne();