Pretty straight forward problem. I simply want to return the affected row created by a SQL INSERT query so I can get data from it.
There's a way I can do it already, I suppose, but I'm hoping the Mysql or Mysql2 gem provides some mechanism for me not having to make the second SELECT query.
The solution I'm leaning towards right now is something akin to:
"INSERT INTO table (col1) VALUES ('value');"
and then:
"SELECT cid FROM table ORDER BY cid DESC LIMIT 1;"
Since cid is the auto-increment index of the table (it's InnoDB fyi), it will always be the largest cid value in the table until you do another INSERT.
Is there any mechanism in Mysql or Mysql2 to avoid having to make that second SELECT query?
MySQL2 has a last_id method. The documentation for that method is worthless but the implementation looks like this:
static VALUE rb_mysql_client_last_id(VALUE self) {
GET_CLIENT(self);
REQUIRE_OPEN_DB(wrapper);
return ULL2NUM(mysql_insert_id(wrapper->client));
}
And the MySQL mysql_insert_id function does this:
Returns the value generated for an AUTO_INCREMENT column by the previous INSERT or UPDATE statement.
So you can do your INSERT and then get the last ID by calling the last_id method.
And BTW, your current approach:
SELECT cid FROM table ORDER BY cid DESC LIMIT 1;
is not safe if you're in a multiprocess environment, consider this:
You INSERT a row.
Another process INSERTs a row.
You SELECT cid FROM table ORDER BY cid DESC LIMIT 1 and get the ID from (2).
You could SELECT last_insert_id() though, last_insert_id() is session-specific so you don't have to worry about other processes when using it.
Related
I want to update the columns and select the values using single query in MySQL
For example
update table set address='bangalore',updated_count =updated_count+1 where id=1
select * from table where id=1
This is not possible. As #D-Shih mentioned, you cannot do both update and select in the same query. The SQL update convention doesn't allow for return data and the select statement can't write to a table.
Each has its own purpose and can't be written in one statement. They must be executed separately.
i'm using MySQL and i want to check if a record exists and if it exists delete this record.
i try this but it 's not working for me:
SELECT 'Barcelone' AS City, EXISTS(SELECT 1 FROM mytable WHERE City = 'Barcelone') AS 'exists';
THEN
DELETE FROM mytable
WHERE City = 'Barcelone';
Thank you for your help.
The if statement is only allowed in stored procedures, stored functions, and triggers (in MySQL).
If I understand what you want, just do:
DELETE FROM mytable
WHERE City = 'Barcelone';
There is no reason to check for the existence beforehand. Just delete the row. If none exist, no problem. No errors.
I would recommend an index on mytable(city) for performance reasons. If you want to check if the row exists first, that is fine, but it is unnecessary for the delete.
If you mean MySQL is returning an error message (if that's what you mean by "not working for me"), then that's exactly the behavior we would expect.
That SQL syntax is not valid for MySQL.
If you want to delete rows from a table, issue a DELETE statement, e.g.
DELETE FROM mytable WHERE City = 'Barcelone'
If you want to know how many rows were deleted (if the statement doesn't throw an error), immediately follow the DELETE statement (in the same session) with a query:
SELECT ROW_COUNT()
Or the appropriate function in whatever client library you are using.
If the ROW_COUNT() function returns 0, then there were no rows deleted.
There's really no point (in terms of MySQL) in issuing a SELECT to find out if there are rows to be deleted; the DELETE statement itself will figure it out.
If for some reason your use case requires you to check whether there are rows be be deleted, then just run a separate SELECT:
SELECT COUNT(1) FROM mytable WHERE City = 'Barcelone'
I'm seeing a weird behavior when I INSERT some data into a table and then run a SELECT query on the same table. This table has an auto-increment primary key (uid), and this problem occurs when I try to then select results where 'uid IS NULL'.
I've golfed this down to the following SQL commands:
DROP TABLE IF EXISTS test_users;
CREATE TABLE test_users (uid INTEGER PRIMARY KEY AUTO_INCREMENT, name varchar(20) NOT NULL);
INSERT INTO test_users(name) values('foo');
SELECT uid FROM test_users WHERE uid IS NULL;
SELECT uid FROM test_users WHERE uid IS NULL; -- no output from this query
I'd expect SELECT uid FROM test_users WHERE uid IS NULL to never return anything, but it does, sometimes. Here's what I've found:
Version of MySQL/MariaDB seems to matter. The machine having this problem is running MySQL 5.1.73 (CentOS 6.5, both 32-bit and 64-bit). My other machine running 5.5.37-MariaDB (Fedora 19, 64-bit). Both running default configs, aside from being configured to use MyISAM tables.
Only the first SELECT query after the INSERT is affected.
If I specify a value for uid rather than let it auto-increment, then it's fine.
If I disconnect and reconnect between the INSERT and SELECT, then I get the expected no results. This is easiest to see in something like Perl where I manage the connection object. I have a test script demonstrating this at https://gist.github.com/avuserow/1c20cc03c007eda43c82
This behavior is by design.
It's evidently equivalent to SELECT * FROM t1 WHERE id = LAST_INSERT_ID(); which would also work only from the connection where you just did the insert, exactly as you described.
It's apparently a workaround that you can use in some environments that make it difficult to fetch the last inserted (by your connection) row's auto-increment value in a more conventional way.
To be precise, it's actually the auto_increment value assigned to the first row inserted by your connection's last insert statement. That's the same thing when you only inserted one row, but it's not the same thing when you insert multiple rows with a single insert statement.
http://dev.mysql.com/doc/connector-odbc/en/connector-odbc-usagenotes-functionality-last-insert-id.html
I have created a routine which inserts a record in one table, and after that it searches for that id (with a select statement) and updates another table's field with that id.Is this possible? It's one routine so my question is if the statements are executed in a sequential order?
Thanks in advance
You do not need to search for the id. You can use LAST_INSERT_ID() to get the id of the last inserted row.
INSERT INTO tablename
(<columns>)
VALUES
(<columnvalues>);
SELECT LAST_INSERT_ID() INTO somevariable;
Then you can write your UPDATE statement.
UPDATE sometable
SET sometable.col = somevariable
WHERE sometable.something = #something
Statements in a routine are executed in the order they are written.
Is there any way to select a record and update it in a single query?
I tried this:
UPDATE arrc_Voucher
SET ActivatedDT = now()
WHERE (SELECT VoucherNbr, VoucherID
FROM arrc_Voucher
WHERE ActivatedDT IS NULL
AND BalanceInit IS NULL
AND TypeFlag = 'V'
LIMIT 1 )
which I hoped would run the select query and grab the first record that matches the where clause, the update the ActivatedDT field in that record, but I got the following error:
1241 - Operand should contain 1 column(s)
Any ideas?
How about:
UPDATE arrc_Voucher
SET ActivatedDT = NOW()
WHERE ActivatedDT IS NULL
AND BalanceInit IS NULL
AND TypeFlag = 'V'
LIMIT 1;
From the MySQL API documentation :
UPDATE returns the number of rows that were actually changed
You cannot select a row and update it at the same time, you will need to perform two queries to achieve it; fetch your record, then update it.
If you are worrying about concurrent processes accessing the same row through some kind of race condition (supposing your use case involve high traffic), you may consider other alternatives such as locking the table (note that other processes will need to recover--retry--if the table is locked while accessing it)
Or if you can create stored procedure, you may want to read this article or the MySQL API documentation.
But about 99% of the time, this is not necessary and the two queries will execute without any problem.