I'd like to limit the entries in a table. Let's say in table tBoss. Is there a SQL constraint that checks how many tuples are currently in the table? Like
SELECT COUNT(*) from tBoss < 2
Firebird says:
Invalid token.
Dynamic SQL Error.
SQL error code = -104.
Token unknown - line 3, column 8.
SELECT.
You could do this with a check constraint and a scalar function. Here's how I built a sample.
First, create a table:
CREATE TABLE MyTable
(
MyTableId int not null identity(1,1)
,MyName varchar(100) not null
)
Then create a function for that table. (You could maybe add the row count limit as a parameters if you want more flexibility.)
CREATE FUNCTION dbo.MyTableRowCount()
RETURNS int
AS
BEGIN
DECLARE #HowMany int
SELECT #HowMany = count(*)
from MyTable
RETURN #HowMany
END
Now add a check constraint using this function to the table
ALTER TABLE MyTable
add constraint CK_MyTable__TwoRowsMax
check (dbo.MyTableRowCount() < 3)
And test it:
INSERT MyTable (MyName) values ('Row one')
INSERT MyTable (MyName) values ('Row two')
INSERT MyTable (MyName) values ('Row three')
INSERT MyTable (MyName) values ('Row four')
A disadvantage is that every time you insert to the table, you have to run the function and perform a table scan... but so what, the table (with clustered index) occupies two pages max. The real disadvantage is that it looks kind of goofy... but everything looks goofy when you don't understand why it has to be that way.
(The trigger solution would work, but I like to avoid triggers whenever possible.)
Does your database have triggers? If so, Add a trigger that rolls back any insert that would add more than 2 rows...
Create Trigger MyTrigName
For Insert On tBoss
As
If (Select Count(*) From tBoss) > 2
RollBack Transaction
but to answer your question directly, the predicate you want is to just put the select subquery inside parentheses. like this ...
[First part of sql statement ]
Where (SELECT COUNT(*) from tBoss) < 2
To find multiples in a database your best bet is a sub-query for example: (Note I am assuming you are looking to find duplicated rows of some sort)
SELECT id FROM tBoss WHERE id IN ( SELECT id FROM tBoss GROUP BY id HAVING count(*) > 1 )
where id is the possibly duplicated column
SELECT COUNT(*) FROM tBoss WHERE someField < 2 GROUP BY someUniqueField
Related
I am looking to insert or update values in an SQLite database (version > 3.35) avoiding multiple queries. upsert along with returning seems promising :
CREATE TABLE phonebook2(
name TEXT PRIMARY KEY,
phonenumber TEXT,
validDate DATE
);
INSERT INTO phonebook2(name,phonenumber,validDate)
VALUES('Alice','704-555-1212','2018-05-08')
ON CONFLICT(name) DO UPDATE SET
phonenumber=excluded.phonenumber,
validDate=excluded.validDate
WHERE excluded.validDate>phonebook2.validDate RETURNING name;
This helps me track names corresponding to inserted/modified rows. How to find rows where phonebook2 values conflict with values upserted in above statement, but no insert or update happened due to where clause?
The RETURNING clause can't be used to get non-affected rows.
What you can do is execute a SELECT statement before the UPSERT:
WITH cte(name, phonenumber, validDate) AS (VALUES
('Alice', '704-555-1212', '2018-05-08'),
('Bob','804-555-1212', '2018-05-09')
)
SELECT *
FROM phonebook2 p
WHERE EXISTS (
SELECT *
FROM cte c
WHERE c.name = p.name AND c.validDate <= p.validDate
);
In the CTE you may include as many tuples as you want
I am writing a query to import data from one table to a new table. I need to insert records that do not exist in the new table, and update records that do exist. I am trying to use a MERGE "upsert" method.
I have some unique problems due to the client's database and application structure. The table I am inserting into has a Unique ID field that increments by 1 for each new row, but the table does not do the auto incrementating; the insert statement needs to pull the highest ID in the target table and add 1 for the new record.
From my research, I can't figure out how to do that with MERGE. I do not database permissions to create a sequence. I have tried a lot of things, but currently my query looks like:
MERGE
dbo.targetTable as target
USING
dbo.sourceTable AS source
ON
target.account_no = source.account_ID
WHEN NOT MATCHED THEN
INSERT (
ID,
FIELD1,
FIELD2,
FIELD3
) VALUES (
(SELECT MAX(ID) + 1 FROM dbo.targetTable),
'field1',
'field2',
'field3'
)
The problem I am then running into with this code is that it appears to only run the select statement for the new ID once. That is, if the highest ID in the target table was 10, it would insert every new record with ID 11. That won't work as I'm getting a
Violation of PRIMARY KEY constraint. Cannot insert duplicate key in object error. I've been doing a ton of googling and trying different things but haven't been able to figure this one out. Any help is appreciated, thank you.
EDIT: For clarification, the unique ID column does not auto-populate. If I do not insert a value for the ID column, I get
Cannot insert the value NULL into column 'ID', table 'dbo.targetTable'; column does not allow nulls. UPDATE fails.
And again, as I mentioned originally I do not have permissions to create sequences. It just throws an error and says I do not have permission to do that.
I agree that changing the ID column to auto-increment automatically would be perfect, but I do not have the capability to modify the table like that either.
If you don't need the IDs to be consecutive, you can add the last available ID to a ROW_NUMBER() to generate new, non-repeated IDs.
BEGIN TRANSACTION
DECLARE #NextAvailableID INT = (SELECT ISNULL(MAX(ID), 0) FROM dbo.targetTable WITH (TABLOCKX))
;WITH SourceWithNewIDs AS
(
SELECT
S.*,
NewID = #NextAvailableID + ROW_NUMBER() OVER (ORDER BY S.account_ID)
FROM
dbo.sourceTable AS S
)
MERGE
dbo.targetTable as target
USING
SourceWithNewIDs AS source
ON
target.account_no = source.account_ID
WHEN NOT MATCHED THEN
INSERT (
ID,
FIELD1,
FIELD2,
FIELD3
) VALUES (
NewID,
'field1',
'field2',
'field3'
)
COMMIT
Keep in mind that this example is missing the proper error handling with rollback and the lock used to retrieve the max ID will block all other operations until commited or rollbacked.
If you need the new rows to have consecutive IDs then you can use this same approach with a regular INSERT (with WHERE NOT EXISTS...) instead of a MERGE (will have to write the UPDATE separately).
This is just a different way without using a Merge. Permissions aren't required for temp tables, so I would use one to hold the account numbers that need to be inserted, with an identity field to help with traversal. A while loop can traverse the identity, inserting the values with respect to the source table's account_no- into the target table. Since the insert is done in a loop, the MAX function should grab the target table's MAX(account_no) correctly on each loop.
DECLARE #tempTable TABLE (pkindex int IDENTITY(1,1) PRIMARY KEY, account_no int)
DECLARE #current int = 1
,#endcount int = 0
--account_no's that should be inserted
INSERT INTO #tempTable(account_no)
SELECT account_no
FROM sourceTable
WHERE account_no NOT IN (SELECT account_no FROM targetTable)
SET #endcount = (SELECT COUNT(*) FROM #tempTable)
--looping condition, should select the MAX(ID) with each subsequent loop
WHILE (#endcount > 0) AND (#current <= #endcount)
BEGIN
INSERT INTO dbo.targetTable(ID, FIELD1, FIELD2, FIELD3)
SELECT (SELECT MAX(T2.ID) + 1 FROM dbo.targetTable T2) AS MAXID
,S.field1
,S.field2
,S.field3
FROM #tempTable T INNER JOIN sourceTable S ON T.account_no = S.account_no
WHERE T.pkindex = #current --traversing temp table by its identity
SET #current += 1
END
Update:
After narrowing down the code it seems that the line
INSERT INTO table1 TABLE table1_temp RETURNING id
is causing the issue. Any tips what is wrong with this?
Original question:
table1 has many colums (I don't care which) and it has an auto increment primary key (id). This is what I need to do and how I'm trying:
First, I'd like to duplicate a record in table1.
BEGIN;
CREATE TEMP TABLE table1_temp ON COMMIT DROP AS
SELECT * FROM table1 WHERE id = <some integer>;
ALTER TABLE table1_temp DROP COLUMN id;
WITH generated_id AS (
INSERT INTO table1 TABLE table1_temp RETURNING id
)
Then, perform an insert to some_table where I need to use the generated id of the copy that was created in table1.
INSERT INTO some_table (something, the_id_into_this)
VALUES ('some value', (SELECT id FROM generated_id));
Then get some data from yet_another_table (columns: somestuff, id_here) and use this and the id for an insert into that same table.
INSERT INTO yet_another_table
(SELECT somestuff,
(SELECT id FROM generated_id) AS id_here
FROM yet_another_table
WHERE id_here = <some integer>)
Finally, I need to return the id so I can use it in my app...
RETURNING id_here AS id;
COMMIT;
Am I on the right path implementing this? When running the query, I get the following error:
column "id" is of type integer but expression is of type character
varying HINT: You will need to rewrite or cast the expression.
It doesn't tell me the line number where it occurrs and I have no idea what might cause this.
INSERT INTO table1 TABLE table1_temp
You cannot do that because table1_temp has different set of columns (you dropped id column).
You need to specify columns explicitly (all but id column):
INSERT INTO table1(col1, col2, ...) TABLE table1_temp
I found a simple solution for cloning a record with an auto increment id that doesn't require you to specify any other columns of the table:
BEGIN;
CREATE TEMP TABLE table1_temp ON COMMIT DROP AS
SELECT * FROM table1 WHERE id = #;
UPDATE table1_temp SET id = nextval('table1_seq');
INSERT INTO table1 TABLE table1_temp;
COMMIT;
And for the CTE part of the question, here is how you can reuse a returned value at multiple subsequent queries by concatenating WITH statements:
WITH generated_id AS (
INSERT INTO ... RETURNING id
), _ AS (
QUERY1 ... SELECT id FROM generated_id ...
), __ AS (
QUERY2 ... SELECT id FROM generated_id ...
...
There is a db2 database with two tables. The first one, table1, has autoincrement column ID. It is the foreign key for the table2.
A am writing an HTML generator for SQL queries. So with some input parameters it generates a query or multiple queries. It is not connected to the database.
What I need is to get that autoincrement field and use it in next queries.
So basically, the scenario is:
insert into table1;
select autogenerated field ID;
insert into table2 using that ID;
insert into table2 using that ID;
...some more similar inserts...
insert into table2 using that ID;
And all that SQL query should be generated and then used as a single SQL script.
I was thinking about something like this:
SELECT ID FROM FINAL TABLE (INSERT INTO Table1 (t1column1, t1column2, etc.)
VALUES (t1value1, t1value2, etc.))
But I don't know, how I can write the result into a variable so I could use it in next queries like this:
INSERT INTO Table2 (foreignKeyCol, t2column1, t2column2, etc.)
VALUES ($ID, t2value1, t2value2, etc.)
I could just paste that select instead of $ID, but the second query can be used several times with the same $ID and different values.
EDIT: DB2 10.5 on Linux.
You can chain several inserts together using CTEs, like so:
WITH idcte (id) as (
SELECT ID FROM FINAL TABLE (
INSERT INTO Table1 (t1column1, t1column2, etc.)
VALUES (t1value1, t1value2, etc.)
)
),
ins1 (id) as (
SELECT foreignKeyCol FROM FINAL TABLE (
INSERT INTO Table2 (foreignKeyCol, t2column1, t2column2, etc.)
SELECT id, t2value1, t2value2, etc.
FROM idcte
)
),
-- more CTEs
SELECT foreignKeyCol FROM FINAL TABLE (
-- your last INSERT ... SELECT FROM
)
Essentially you will have to wrap each INSERT into a SELECT FROM FINAL TABLE for this to work.
Alternatively, you can use a global variable to keep the ID value:
CREATE VARIABLE myNewId INT;
SET myNewId = (SELECT ID FROM FINAL TABLE (
INSERT INTO Table1 (t1column1, t1column2, etc.)
VALUES (t1value1, t1value2, etc.)
));
INSERT INTO Table2 (foreignKeyCol, t2column1, t2column2, etc.)
VALUES (myNewId, t2value1, t2value2, etc.);
DROP VARIABLE myNewId;
This assumes a recent version of Db2 for LUW.
So, I'm looking at a stored procedure here, which has more than one line like the following pseudocode:
if(select count(*) > 0)
...
on tables having a unique id (or identifier, for making it more general).
Now, in terms of performance, is it more performant to change this clause
to
if(select count([uniqueId]) > 0)
...
where uniqueId is, e.g., an Idx containing double values?
An example:
Consider a table like Idx (double) | Name (String) | Address (String)
Now the 'Idx' is a foreign key which I want to join in a stored procedure.
So, in terms of performance: what is better here?
if(select count(*) > 0)
...
or
if(select count(Idx) > 0)
...
? Or does the SQL Engine Change select count(*) to select count(Idx) internally, so we do not have to bother about this? Because at first sight, I'd say that select count(Idx) would be more performant.
The two are slightly different. count(*) counts rows. count([uniqueid]) counts the number of non-NULL values for uniqueid. Because a unique constraint allows a NULL value, SQL Server actually needs to read the column. This could add microseconds of time to a query, particularly if the page with the id is not already in memory. This also gives SQL Server more opportunities to optimize count(*).
As #lad2025 writes in a comment, the performant solution is to use if (exists . . ..
SELECT t1.*
FROM Table1 t1
JOIN Table2 t2 ON t2.idx = t1.idx
will give you only the rows in t1 that match an idx value in Table2. I'm not sure there is a good reason to do an if(select count...).
If you are really interested in the performance of something like this, just create a temp table with a million rows and give it a go:
CREATE TABLE #TempTable (id int identity, txt varchar(50))
GO
INSERT #TempTable (txt) VALUES (##IDENTITY)
GO 1000000