I'm trying to avoid writing separate SQL queries to achieve the following scenario:
I have a Table called Values:
Values:
id INT (PK)
data TEXT
I would like to check if certain data exists in the table, and if it does, return its id, otherwise insert it and return its id.
The (very) naive way would be:
select id from Values where data = "SOME_DATA";
if id is not null, take it.
if id is null then:
insert into Values(data) values("SOME_DATA");
and then select it again to see its id or use the returned id.
I am trying to make the above functionality in one line.
I think I'm getting close, but I couldn't make it yet:
So far I got this:
select id from Values where data=(COALESCE((select data from Values where data="SOME_DATA"), (insert into Values(data) values("SOME_DATA"));
I'm trying to take advantage of the fact that the second select will return null and then the second argument to COALESCE will be returned. No success so far. What am I missing?
Your command does not work because in SQL, INSERT does not return a value.
If you have a unique constraint/index on the data column, you can use that to prevent duplicates if you blindly insert the value; this uses SQLite's INSERT OR IGNORE extension:
INSERT OR IGNORE INTO "Values"(data) VALUES('SOME_DATE');
SELECT id FROM "Values" WHERE data = 'SOME_DATA';
Related
I have a table with ID primary key (autoincrement) and a unique column Name. Is there an efficient way in MariaDB to insert a row into this table if the same Name doesn't exist, otherwise select the existing row and, in both cases, return the ID of the row with this Name?
Here's a solution for Postgres. However, it seems MariaDB doesn't have the RETURNING id clause.
What I have tried so far is brute-force:
INSERT IGNORE INTO services (Name) VALUES ('JohnDoe');
SELECT ID FROM services WHERE Name='JohnDoe';
UPDATE: MariaDB 10.5 has RETURNING clause, however, the queries I have tried so far throw a syntax error:
WITH i AS (INSERT IGNORE INTO services (`Name`) VALUES ('John') RETURNING ID)
SELECT ID FROM i
UNION
SELECT ID FROM services WHERE `Name`='John'
For a single row, assuming id is AUTO_INCREMENT.
INSERT INTO t (name)
VALUES ('JohnDoe')
ON DUPLICATE KEY id = LAST_INSERT_ID(id);
SELECT LAST_INSERT_ID();
That looks kludgy, but it is an example in the documentation.
Caution: Most forms of INSERT will "burn" auto_inc ids. That is, they grab the next id(s) before realizing that the id won't be used. This could lead to overflowing the max auto_inc size.
It is also wise not to put the normalization inside the transaction that does the "meat" of the code. It ties up the table unnecessarily long and runs extra risk of burning ids in the case of rollback.
For batch updating of a 'normalization' table like that, see my notes here: http://mysql.rjweb.org/doc.php/staging_table#normalization (It avoids burning ids.)
I have a table with PK and another column for other id. In some cases i need to insert record with equal values in both columns. For primary key values i'm using sequence, which gives a Field<Long> from Sequences.MY_SEQ.nextval().
How can i extract value from a Field<Long> for guaranteed insert same ids in both columns? Using Field<Long> in insert clause generates 2 different ids in columns.
Here is the solution:
Long id = dsl.select(Sequences.MY_SEQ.nextval()).fetchOne().value1();
Your own solution works, of course, but it will generate two round trips to the database. One for fetching the sequence value and another one for the insert. If that's not a problem, perfect. Otherwise, you can still do it in one single query using INSERT .. SELECT:
In SQL:
(using Oracle syntax. Your SQL syntax may vary...)
INSERT INTO my_table (col1, col2, val)
SELECT t.id, t.id, 'abc'
FROM (
SELECT my_seq.nextval AS id
FROM dual
) t
With jOOQ
Table<?> t = table(select(MY_SEQ.nextval().as("id"))).as("t");
dsl.insertInto(MY_TABLE)
.columns(MY_TABLE.COL1, MY_TABLE.COL2, MY_TABLE.VAL)
.select(
select(t.field("id"), t.field("id"), val("abc"))
.from(t))
.execute();
For a table like this one:
CREATE TABLE Users(
id SERIAL PRIMARY KEY,
name TEXT UNIQUE
);
What would be the correct one-query insert for the following operation:
Given a user name, insert a new record and return the new id. But if the name already exists, just return the id.
I am aware of the new syntax within PostgreSQL 9.5 for ON CONFLICT(column) DO UPDATE/NOTHING, but I can't figure out how, if at all, it can help, given that I need the id to be returned.
It seems that RETURNING id and ON CONFLICT do not belong together.
The UPSERT implementation is hugely complex to be safe against concurrent write access. Take a look at this Postgres Wiki that served as log during initial development. The Postgres hackers decided not to include "excluded" rows in the RETURNING clause for the first release in Postgres 9.5. They might build something in for the next release.
This is the crucial statement in the manual to explain your situation:
The syntax of the RETURNING list is identical to that of the output
list of SELECT. Only rows that were successfully inserted or updated
will be returned. For example, if a row was locked but not updated
because an ON CONFLICT DO UPDATE ... WHERE clause condition was not
satisfied, the row will not be returned.
Bold emphasis mine.
For a single row to insert:
Without concurrent write load on the same table
WITH ins AS (
INSERT INTO users(name)
VALUES ('new_usr_name') -- input value
ON CONFLICT(name) DO NOTHING
RETURNING users.id
)
SELECT id FROM ins
UNION ALL
SELECT id FROM users -- 2nd SELECT never executed if INSERT successful
WHERE name = 'new_usr_name' -- input value a 2nd time
LIMIT 1;
With possible concurrent write load on the table
Consider this instead (for single row INSERT):
Is SELECT or INSERT in a function prone to race conditions?
To insert a set of rows:
How to use RETURNING with ON CONFLICT in PostgreSQL?
How to include excluded rows in RETURNING from INSERT ... ON CONFLICT
All three with very detailed explanation.
For a single row insert and no update:
with i as (
insert into users (name)
select 'the name'
where not exists (
select 1
from users
where name = 'the name'
)
returning id
)
select id
from users
where name = 'the name'
union all
select id from i
The manual about the primary and the with subqueries parts:
The primary query and the WITH queries are all (notionally) executed at the same time
Although that sounds to me "same snapshot" I'm not sure since I don't know what notionally means in that context.
But there is also:
The sub-statements in WITH are executed concurrently with each other and with the main query. Therefore, when using data-modifying statements in WITH, the order in which the specified updates actually happen is unpredictable. All the statements are executed with the same snapshot
If I understand correctly that same snapshot bit prevents a race condition. But again I'm not sure if by all the statements it refers only to the statements in the with subqueries excluding the main query. To avoid any doubt move the select in the previous query to a with subquery:
with s as (
select id
from users
where name = 'the name'
), i as (
insert into users (name)
select 'the name'
where not exists (select 1 from s)
returning id
)
select id from s
union all
select id from i
In DB2 I can do a command that looks like this to retrieve information from the inserted row:
SELECT *
FROM NEW TABLE (
INSERT INTO phone_book
VALUES ( 'Peter Doe','555-2323' )
) AS t
How do I do that in Postgres?
There are way to retrieve a sequence, but I need to retrieve arbitrary columns.
My desire to merge a select with the insert is for performance reasons. This way I only need to execute one statement to insert values and select values from the insert. The values that are inserted come from a subselect rather than a values clause. I only need to insert 1 row.
That sample code was lifted from Wikipedia Insert Article
A plain INSERT ... RETURNING ... does the job and delivers best performance.
A CTE is not necessary.
INSERT INTO phone_book (name, number)
VALUES ( 'Peter Doe','555-2323' )
RETURNING * -- or just phonebook_id, if that's all you need
Aside: In most cases it's advisable to add a target list.
The Wikipedia page you quoted already has the same advice:
Using an INSERT statement with RETURNING clause for PostgreSQL (since
8.2). The returned list is identical to the result of a SELECT.
PostgreSQL supports this kind of behavior through a returning clause in a common table expression. You generally shouldn't assume that something like this will improve performance simply because you're executing one statement instead of two. Use EXPLAIN to measure performance.
create table test (
test_id serial primary key,
col1 integer
);
with inserted_rows as (
insert into test (c1) values (3)
returning *
)
select * from inserted_rows;
test_id col1
--
1 3
Docs
I want something like this. But I dont know the proper syntax.
select id from table1
where name = (insert into table1 (name) values ('value_abcd'))
So you have a table with an autoincrementing primary key, and you want to know its value after you have inserted a record.
In SQL, you can use the last_insert_rowid function:
INSERT INTO table1(name) VALUES('value_abcd');
SELECT last_insert_rowid();
However, in most cases you do not need to execute a separate SELECT query because most libraries have some function to return that value.
For example, SQLite's C API has the sqlite3_last_insert_rowid function, and Android's insert function returns the ID directly.
When you say:
WHERE something = somethingElse
somethingElse is an expression (so is something, but anyway). You are trying to use an INSERT statement as your somethingElse, but an INSERT statement doesn't return anything, so it can not be evaluated as an expression. Therefore, not only is whatever you're trying to invalid syntax, it also doesn't make any sense in a pseudo-code kind of way.
Perhaps if you can try to explain more clearly what it is you're trying to do (and why), someone will be able to suggest something for you.
If id column of the table is set as identity, then in SQL
insert into table1(name) value('name')
select SCOPE_IDENTITY()
It will return last auto generated idetity value
If id column is not set as identity then in SQL
insert into table1(name)
output inserted.id
value('name')
It will return all id values inserted in a single statement.