My table has two columns: "id" (Auto Increment, Primary) and "number" (Unique). Now I want to the following:
if the number already exists, return the id;
else, add entry to the table and return its id.
What's the most efficient method to do this job?
Note:
There is a greater probability that the number is new;
The table will contain hundreds of thousands of records.
Thank you!
INSERT IGNORE INTO table (number) VALUES (42);
SELECT id FROM table WHERE number = 42;
That's probably the most efficient in MySQL. You could use a Stored Procedure to lump them up, which may or may not be slightly more efficient.
EDIT:
If you think it's going to be rare that new numbers come up, this will be even faster:
SELECT id FROM table WHERE number = 42;
if (!id) {
INSERT INTO table WHERE number = 42;
id = SELECT #LAST_INSERT_ID;
}
There is a possible race condition here if concurrent threads simultaneously select then insert the same number at the same time. In this case, the later insert will fail. You could recover from this by re-selecting on this error condition.
Here is one such stored function that does what you describe:
CREATE FUNCTION `spNextNumber`(pNumber int) RETURNS int(11)
BEGIN
DECLARE returnValue int;
SET returnValue := (SELECT Number FROM Tbl WHERE Number = pNumber LIMIT 1);
IF returnValue IS NULL THEN
INSERT IGNORE INTO Tbl (Number) VALUES (pNumber);
SET returnValue := pNumber; -- LAST_INSERT_ID() can give you the real, surrogate key
END IF;
RETURN returnValue;
END
I know this is old, but it is a common problem. So, for the sake of anyone searching for a solution here are 4 different ways to accomplish this task with performance benchmarks. http://mikefenwick.com/blog/insert-into-database-or-return-id-of-duplicate-row-in-mysql/.
Related
Let's say I create a simple table and a stored function to populate it:
CREATE TABLE t
(
id INT PRIMARY KEY,
val VARCHAR(255)
)//
CREATE FUNCTION populateIfNotAlreadyIn (idArg INT, valArg VARCHAR(255))
RETURNS BOOLEAN NOT DETERMINISTIC
BEGIN
DECLARE alreadyIn INT;
SELECT COUNT(id) INTO alreadyIn FROM t WHERE id = idArg;
IF alreadyIn <> 0 THEN
RETURN FALSE;
ELSE
INSERT INTO t (id, val) VALUES (idArg, valArg);
RETURN TRUE;
END IF;
END//
Of course, the intended behaviour is that the function returns 0 if a row with the given ID already exists in the table, and 1 if it doesn't (in the latter case, the function also inserts such a row).
So now testing comes.
If I run testing queries in phpmyadmin one by one like that:
SELECT populateIfNotAlreadyIn(1, 'val1');
//Go//
SELECT populateIfNotAlreadyIn(2, 'val2');
//Go//
… etc
Then apparently, the results are correct: the function returns 1 and inserts a row if and only if there is no row with such an id in the table, otherwise it returns 0.
So far, so good. But instead of running queries one by one, let's feed phpmyadmin with all of them at once. To be more specific, let's insert into phpmyadmin sth like this:
SELECT populateIfNotAlreadyIn(1, 'val1');
SELECT populateIfNotAlreadyIn(2, 'val2');
SELECT populateIfNotAlreadyIn(3, 'val3');
SELECT populateIfNotAlreadyIn(4, 'val4');
SELECT populateIfNotAlreadyIn(5, 'val5');
And then click Go, to run all of them.
Result? The rows are inserted correctly into table t. But the stored function returns incorrect values. Each call except the last one returns a 0.
Why does this happen?
I have a table with 3 fields: Id(PK,AI), Name(varchar(36)), LName(varchar(36)).
I have to insert name and last name, Id inserts automatically because of it's constraints,
Is There a way to Jump id auto increment value when it reaches 6?
for instance do this 7 times:
Insert Into table(Name, LName) Values ('name1', 'lname1') "And jump id to 7 if it is going to be 6"
It may sound stupid to do this but I have the doubt.
Also Jump and do not record id 6.
record only, 1-5, 7,8,9 and so on
What I want to achieve starts from a Union:
Select * From TableNames
Union All
Select * From TableNames_general
In the TableNames_general I assign it's first value so that when the user sees the table for the first time it will be displayed the record I inserted.
The problem comes when the user inserts a new record, if the Id of the inserted record is the same as the one I have inserted it will be duplicated, that is why I want to achieve when the users inserts one record and if the last insert id already exists just jump that record. this is because I must have different ids due to its relationship among child tables.
Identity column generate values for you, And its best left this way, You have the ability to insert specific values in Identity column but its best left alone and let it generate values for you.
Imagine you have inserted a value explicitly in an identity column and then later on Identity column generates the same value for you, you will end up with duplicates.
If you want to have your input in that column then why bother with identity column anyway ??
Well this is not the best practice but you can jump to a specific number by doing as follows:
MS SQL SERVER 2005 and Later
-- Create test table
CREATE TABLE ID_TEST(ID INT IDENTITY(1,1), VALUE INT)
GO
-- Insert values
INSERT INTO ID_TEST (VALUE) VALUES
(1),(2),(3)
GO
-- Set idnentity insert on to insert values explicitly in identity column
SET IDENTITY_INSERT ID_TEST ON;
INSERT INTO ID_TEST (ID, VALUE) VALUES
(6, 6),(8,8),(9,9)
GO
-- Set identity insert off
SET IDENTITY_INSERT ID_TEST OFF;
GO
-- 1st reseed the value of identity column to any smallest value in your table
-- below I reseeded it to 0
DBCC CHECKIDENT ('ID_TEST', RESEED, 0);
-- execute the same commad without any seed value it will reset it to the
-- next highest idnetity value
DBCC CHECKIDENT ('ID_TEST', RESEED);
GO
-- final insert
INSERT INTO ID_TEST (VALUE) VALUES
(10)
GO
-- now select data from table and see the gap
SELECT * FROM ID_TEST
If you query the database to get the last inserted ID, then you can check if you need to increment it, by using a parameter in the query to set the correct ID.
If you use MSSQL, you can do the following:
Before you insert check for the current ID, if it's 5, then do the following:
Set IDENTITY_INSERT to ON
Insert your data with ID = 7
Set IDENTITY_INSERT to OFF
Also you might get away with the following scenario:
check for current ID
if it's 5, run DBCC CHECKIDENT (Table, reseed, 6), it will reseed the table and in this case your next identity will be 7
If you're checking for current identity just after INSERT, you can use SELECT ##IDENTITY or SELECT SCOPE_IDENTITY() for better results (as rcdmk pointed out in comments)
Otherwise you can just use select: SELECT MAX(Id) FROM Table
There's no direct way to influence the AUTO_INCREMENT to "skip" a particular value, or values on a particular condition.
I think you'd have to handle this in an AFTER INSERT trigger. An AFTER INSERT trigger can't update the values of the row that was just inserted, and I don't think it can make any modifications to the table affected by the statement that fired the trigger.
A BEFORE INSERT trigger won't work either, because the value assigned to an AUTO_INCREMENT column is not available in a BEFORE INSERT trigger.
I don't believe there's a way to get SQL Server IDENTITY to "skip" a particular value either.
UPDATE
If you need "unique" id values between two tables, there's a rather ugly workaround with MySQL: roll your own auto_increment behavior using triggers and a separate table. Rather than defining your tables with AUTO_INCREMENT attribute, use a BEFORE INSERT trigger to obtain a value.
If an id value is supplied, and it's larger than the current maximum value from the auto_increment column in the dummy auto_increment_seq table, we'd need to either update that row, or insert a new one.
As a rough outline:
CREATE TABLE auto_increment_seq
(id INT NOT NULL PRIMARY KEY AUTO_INCREMENT) ENGINE=MyISAM;
DELIMITER $$
CREATE TRIGGER TableNames_bi
BEFORE INSERT ON TableNames
FOR EACH ROW
BEGIN
DECLARE li_new_id INT UNSIGNED;
IF ( NEW.id = 0 OR NEW.id IS NULL ) THEN
INSERT INTO auto_increment_seq (id) VALUES (NULL);
SELECT LAST_INSERT_ID() INTO li_new_id;
SET NEW.id = li_new_id;
ELSE
SELECT MAX(id) INTO li_max_seq FROM auto_increment_seq;
IF ( NEW.id > li_max_seq ) THEN
INSERT INTO auto_increment_seq (id) VALUES (NEW.id);
END IF;
END IF;
END$$
CREATE TRIGGER TableNames_ai
AFTER INSERT ON TableNames
FOR EACH ROW BEGIN
DECLARE li_max_seq INT UNSIGNED;
SELECT MAX(id) INTO li_max_seq FROM auto_increment_seq;
IF ( NEW.id > li_max_seq ) THEN
INSERT INTO auto_increment_seq (id) VALUES (NEW.id);
END IF;
END;
DELIMITER ;
The id column in the table could be defined something like this:
TableNames
( id INT UNSIGNED NOT NULL DEFAULT 0 PRIMARY KEY
COMMENT 'populated from auto_increment_seq.id'
, ...
You could create an identical trigger for the other table as well, so the two tables are effectively sharing the same auto_increment sequence. (With less efficiency and concurrency than an Oracle SEQUENCE object would provide.)
IMPORTANT NOTES
This doesn't really insure that the id values between the tables are actually kept unique. That would really require a query of the other table to see if the id value exists or not; and if running with InnoDB engine, in the context of some transaction isolation levels, we might be querying a stale (as in, consistent from the point in time at the start of the transaction) version of the other table.
And absent some additional (concurrency killing) locking, the approach outline above is subject to a small window of opportunity for a "race" condition with concurrent inserts... the SELECT MAX() from the dummy seq table, followed by the INSERT, allows a small window for another transaction to also run a SELECT MAX(), and return the same value. The best we can hope for (I think) is for an error to be thrown due to a duplicate key exception.
This approach requires the dummy "seq" table to use the MyISAM engine, so we can get an Oracle-like AUTONOMOUS TRANSACTION behavior; if inserts to the real tables are performed in the context of a REPEATABLE READ or SERIALIZABLE transaction isolation level, reads of the MAX(id) from the seq table would be consistent from the snapshot at the beginning of the transaction, we wouldn't get the newly inserted (or updated) values.
We'd also really need to consider the edge case of an UPDATE of row changing the id value; to handle that case, we'd need BEFORE/AFTER UPDATE triggers as well.
I'm trying to create a simple table where I insert field and I do some checks in MySql. I've used Microsoft SQL relatively easy. Instead, MySql give evrrytime query errors without even specifying what's going on. Poor MySql software design apart, here's what I'm trying to do:
1 table with 4 fields with an autoincremental autogenerated number to det an ID as primary key
CREATE TABLE `my_db`.`Patients_table` (
`ID_Patient` INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY ,
`Patient_name` VARCHAR( 200 ) NOT NULL ,
`Recovery_Date` DATETIME NOT NULL ,
`Recovery_count` INT NOT NULL
) ENGINE = MYISAM
a simple stored procedure to insert such fields and check if something exist before inserting:
CREATE PROCEDURE nameInsert(IN nome, IN data)
INSERT INTO Patients_table (Patient_name,Recovery_Date) values (nome,data)
IF (EXISTS (SELECT Recovery_count FROM Tabella_nomi) = 0) THEN
INSERT INTO (Patients_table (Recovery_count)
ELSE
SET Recovery_count = select Recovery_count+1 from Patients_table
END
this seems wrong on many levels and MySQL useless syntax checker does not help.
How can I do this? Thanks.
There seems to be a lot wrong with this block of code. (No offense intended!)
First, Procedures need to be wrapped with BEGIN and END:
CREATE PROCEDURE nameInsert(IN nome, IN data)
BEGIN
...[actually do stuff here]
END
Second, since your table is declared with all fields as NOT NULL, you must insert all fields with an INSERT statement (this includes the Recovery_Date column, and excludes the AUTO_INCREMENT column). You can add DEFAULT CURRENT_TIMESTAMP to the date column if you want it to be set automatically.
INSERT INTO Patients_table (Patient_name,Recovery_Date) values (nome,data)
Third, what exactly is your IF predicate doing?
EXISTS (SELECT Recovery_count FROM Tabella_nomi) = 0
If you want to check if a row exists, don't put the = 0 at the end. Also, Tabella_nomi isn't declared anywhere in that procedure. Also, your SELECT statement should have a WHERE clause, since I'm assuming you want to select a specific row (this is going to select a result set of all recovery_counts).
Fourth, the second INSERT statement seems a little messy. It should look more like the first INSERT, and keep the point I made above in mind.
INSERT INTO (Patients_table (Recovery_count)
Fifth, the ELSE statement
SET Recovery_count = select Recovery_count+1 from Patients_table
Has some problems too. SET is meant for setting variables, not values in rows. I'm not 100% sure what your intent is from this statement, but it looks like you meant to increment the Recovery_count column of a certain row if it already exists. In which case, you meant to do something like this:
UPDATE Patients_table SET Recovery_count = Recovery_count+1 WHERE <conditional predicate>
Where the conditional predicate is something like this:
Patients_name = nome
Try these things, and look at the errors it gives you when you try to execute the CREATE STATEMENT. I bet they're more useful then you think!
I know how LAST_INSERT_ID() works for auto incremented columns, but I cannot find a way to get the last id I inserted for a non auto incremented column.
Is there a way I can do that?
you can easily do that using the same LAST_INSERT_ID().
INSERT INTO thetable (id, value)
VALUES (LAST_INSERT_ID(126), 'some data');
SELECT LAST_INSERT_ID(); -- returns 126
I'm assuming you want the retrieve this last inserted id at some later point after inserting it, since if you need it right after inserting it you obviously would already know what the id is.
The only way you'll be able to get that is to have another column on the table that can indicate which row was last inserted, such as a timestamp or datetime column. If your ids are unique and increasing, you can just use that column. Then you just select 1 row ordered by that column in descending order.
For example
INSERT INTO my_table (id, timestamp) VALUES (123, NOW())
SELECT id FROM my_table ORDER BY timestamp DESC LIMIT 1
Edit: as per the comments below, you're much better off using an AUTO_INCREMENT column, though this column doesn't have to be the id column, you could add an auto-increment insert_order column of type Int and simply order by that.
I assume that you need the ID to find your just inserted row, rather to find the last inserted row. In a web application, you can never be sure that the last inserted row is the one you have just created.
You could use a GUID as id in this case. A GUID is usually stored as a string of length 36 or as a 16byte blob. The GUID can be created before inserting the row, and then can be stored while inserting the row.
Since the id is not auto incremented as you stated, you have to generate it anyway before inserting the row. The safest way to do this is to create a GUID which should be unique enough. Otherwise you would have to determine the last unused ID, what can be tricky and risky.
The easiest way I found to do this is to set a variable.
Unlike using LAST_INSERT_ID which only returns and INT this way you can use other unique identifiers.
SET #id = UUID();
INSERT INTO users (
id
)
VALUES (
#id
);
SELECT * FROM users WHERE id = #id;
No.
There is no inherent ordering of relations, no "last-inserted record". This is why the AUTO_INCREMENT field exists, after all.
You'd have to look in logs or cache the value yourself inside your application.
There's no way with mysql. But you can to do it programmatically. Without an auto-incrementing ID column there's no way for the database to know which records were inserted last.
One way to do is use such as a column containing timestamp or datetime values. and get id of latest value of tmestamp to get last inserted record
If you want to get a custom last_inserted ID, you must implement a procedure that will make the insert statment on your DB.
At the end, just print the ID and use the PHP (if PHP is your main script) sender to return the generated row.
EXAMPLE:
DROP PROCEDURE IF EXISTS insert_row;
DELIMITER $$
CREATE PROCEDURE insert_row(IN _row_id VARCHAR(255), IN _description VARCHAR(255))
BEGIN
SET #last_inserted_id = _row_id;
SET #sql = CONCAT("INSERT INTO test VALUES ('", _row_id, "','",_description,"')");
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SELECT #last_inserted_id AS LAST_INSERT_ID;
END;
$$
DELIMITER ;
#
#
#
#------- HOW TO USE IT ? ---------------
CALL insert_row('Test001','the first test line');
This worked for me in XAMPP
$qry = $con->query("INSERT INTO test_table(tbl_id, txt) VALUES(last_insert_id('15'), 'test value')");
print_r($con->insert_id);
hi I'm quite new to mysql and I'm trying to figure out how to use triggers.
what I'm trying to do:
I have 2 tables, max and sub_max, when I insert a new row to sub_max I want to check if the SUM of the values with the same foreign_key as the new row are less than the value in the max table. I think this sounds confusing so here are my tables:
CREATE TABLE max(
number INT ,
MaxAmount integer NOT NULL)
CREATE TABLE sub_max(
sub_number INT ,
sub_MaxAmount integer NOT NULL,
number INT,
FOREIGN KEY ( number ) REFERENCES max( number ))
and here is my code for the trigger, I know the syntax is off but this is the best I could do from looking up tutorials.
CREATE TRIGGER maxallowed
after insert on sub_max
FOR EACH ROW
BEGIN
DECLARE submax integer;
DECLARE maxmax integer;
submax = select sum(sub_MaxAmount) from sub_max where sub_number = new.sub_number;
submax = submax + new. sub_MaxAmount;
maxmax = select MaxAmount from max where number = new.number ;
if max>maxmax
rollback?
END
I wanted to know if I'm doing this remotely correctly. Thanks in advance.
Caveat - I am also learning triggers.
For the section:
if max>maxmax
rollback?
Would the syntax be something like?:
IF max > maxmax THEN
DELETE the id of the new record?
ELSE
do nothing?
END IF;