I have a table with an auto incremental primary ID, and I sometimes need to insert rows in this table, which have a column that needs to be the same value as the primary ID. How can i set this column to be the same value as the primary ID before the row is inserted? Is this even possible, or would I have to do another update query after the row has been inserted?
The canonical way of referencing a new key is to use LAST_INSERT_ID(). You could try inserting LAST_INSERT_ID() as the value:
INSERT INTO FOO (dependent_field) values (LAST_INSERT_ID());
From http://dev.mysql.com/doc/refman/5.0/en/example-auto-increment.html
You can retrieve the most recent AUTO_INCREMENT value with the LAST_INSERT_ID() SQL function or the mysql_insert_id() C API function. These functions are connection-specific, so their return values are not affected by another connection which is also performing inserts.
Caveat: I have not tested this, so it's quite possible that LAST_INSERT_ID() will not evaluate properly within the same statement as the insert that generates the new ID. If that's the case, the only solution I can think of is to do an update afterwards, as you suggest.
Related
I want to insert multiple rows into a table, using a single INSERT statement. This is no problem, since SQL offers the option to provide multiple rows as parameter for a single INSERT statement. Now, those rows contain an ID field that is incremented automatically, i.e. its value is set by the database, not by my code.
As a result, I would like to get the ID values of the inserted rows. My basic question is: How do I do that for MariaDB / MySQL?
As it turns out, this is pretty simple, e.g. in PostgreSQL, as PostgreSQL has the RETURNING clause for INSERT which returns the desired values for one or even for multiple rows. This is exactly what I want and it works.
Unfortunately, neither MariaDB nor MySQL have PostgreSQL's RETURNING clause, so I need to fallback to something such as LAST_INSERT_ID(), but this only returns the ID of the single last inserted row, even if multiple rows were inserted using a single INSERT. How do I get all the ID values?
My code currently looks like this:
INSERT INTO mytable
(foo, bar)
VALUES
('fooA', 'barA'),
('fooB', 'barB');
SELECT LAST_INSERT_ID() AS id;
How can I solve this issue in a way that works even with concurrent writes?
(And no, it's not an option to change to a UUID field, or something like this; the auto-increment field is given, and can not be changed.)
MySQL & MariaDB have the LAST_INSERT_ID() function, and it returns the id generated by the most recent INSERT statement in your current session.
But when your INSERT statement inserts multiple rows, LAST_INSERT_ID() returns the first id in the set generated.
In such a batch of multiple rows, you can rely on the subsequent id's being consecutive. The MySQL JDBC driver depends on this, for example.
If the rows you insert include a mix of NULL and non-NULL values for the id column, you have a risk of messing up this assumption. The JDBC driver returns the wrong values for the set of generated id's.
As stated in the comments, you can capture the inserted IDs (SQL Server):
use tempdb
create table test (
id int identity(1,1) primary key,
t varchar(10) null
)
create table ids (
i int not null
)
insert test(t)
output inserted.id into ids
values (null), (null), (null)
select *
from test
select *
from ids
So, I have an insert statement to insert data into my database by implode() an array
$sql->prepared("INSERT INTO student(studentId,studentemail,phone,address)VALUE".implode(',', array());
The problem is every time I run the script I have to delete every row from my database because my array already have the values stored in the database plus the new values which I want to insert. So, I want to implement an query in which if existed don't Insert. I am not sure how to play with it.
You can use insert on duplicate key. First you need a unique index. I am guessing:
create unique index unq_student_studentid on student(studentid);
insert into student (studentId, studentemail, phone, address)
value . . .
on duplicate key studentid = values(studentid);
This will ignore existing data. Using the same method, you can also update the other fields, if you prefer the newer values.
That said, inserting the data every time seems like a poorly designed application.
Create a unique index on studentid (it probably should already be the primary key) and then use INSERT IGNORE instead of ordinary INSERT. This just skips any rows that would violate the unique constraint.
I am doing the following SQL tutorial: http://sql.learncodethehardway.org/book/ex11.html
and in this exercise the author says in the second paragraph:
In this situation, I want to replace my record with another guy but
keep the unique id. Problem is I'd have to either do a DELETE/INSERT
in a transaction to make it atomic, or I'd need to do a full UPDATE.
Could anyone explain to me what the problem is with doing an UPDATE, and when we might choose REPLACE instead of UPDATE?
The UPDATE code:
UPDATE person SET first_name = "Frank", last_name = "Smith", age = 100
WHERE id = 0;
Here is the REPLACE code:
REPLACE INTO person (id, first_name, last_name, age)
VALUES (0, 'Frank', 'Smith', 100);
EDIT: I guess another question I have is why would you ever do a DELETE/INSERT instead of just an UPDATE as is discussed in the quoted section?
According to the documentation, the difference is:
REPLACE works exactly like INSERT, except that if an old row in the table has the same value as a new row for a PRIMARY KEY or a UNIQUE index, the old row is deleted before the new row is inserted.
So what it does:
Try to match the row using one of the available indexes;
If the row doesn't exist already: add a new one;
If the row exists already: delete the existing row and add a new one afterwards.
When might using this become useful over separate insert and update statements?
You can safely call this, and you don't have to worry about existing rows (one statement vs. two);
If you want related data to be removed when inserting / updating, you can use replace: it deletes all related data too);
When triggers need to fire, and you expect an insert (bad reason, okay).
First Replace isn't widely understood in all database engines.
Second replace inserts/updates a record based on the primary key. While with update you can specify more elaborate conditions:
UPDATE person SET first_name = 'old ' + first_name WHERE age > 50
Also UPDATE won't create records.
UPDATE will have no effect if the row does not exist.
Where as the INSERT or REPLACE will insert if the row doesn't exists or replace the values if it does.
Update will change the existing records value in table based on particular condition. So you can change one or many records in single query.
Insert or Replace will insert a new record if records is not present in table else will replace. Replace will only work if and only if you provide the primary key value in the insert or replace query. If you forget to add primary key field value than a new record will created in table.
Case example:-
Update: You have a calculation of wages to be done based on a formula using the column values. In this case you will always use update query as using one single query you can update multiple records.
Insert or Replace: Already mentioned in the link you shared.
How the REPLACE INTO statement works:
AS INSERT:
REPLACE INTO table_name (column1name, column2name, ...)
VALUES (value1, value2, ...);
AS UPDATE:
REPLACE INTO table_name SET column1name = value, column2name = value, ... ;
The REPLACE statement checks whether the intended data record's unique key value already exists in the table before inserting it as a new record or updating it.
The REPLACE INTO statement attempts to insert a new record or modify an existing record. In both cases, it checks whether the unique key of the proposed record already exists in the table. Suppose a value of NO or FALSE is returne. In that case, the REPLACE statement inserts the record similar to the INSERT INTO statement.
Suppose the key value already exists in the table (in other words, a duplicate key). In that case, the REPLACE statement deletes the existing record of data and replaces it with a new record of data. This happens regardless of whether you use the first or the second REPLACE statement syntax.
Once the REPLACE INTO statement is used to insert or modify data, it determines first whether the new data record already exists in the table. It checks if the PRIMARY or the UNIQUE KEY matches one of the existing records.
If there is no matching key, the REPLACE works like a normal INSERT statement. Otherwise, it deletes the existing record and replaces it with the new one. This is considered a sort of modification or update of an existing record. However, it would be best if you were careful here. Suppose you do not specify a value for a column in the SET clause. In that case, the REPLACE statement uses the default value (if a default value has been set). Otherwise, it's set as NULL.
I'd like to update a column in my table with the country code and the new id which is an auto-increment value.
BEGIN
SET new.key = concat(new.countryCode,new.id);
END
countryCode works fine, but the id is always blank. How can I achieve something like this? The id comes from an autoincrement column.
I know it's not working because it's generated after the insert. So how can I do something like this?
AUTO_INCREMENT column are only set after insert.
If you need to access that value, you can only in an AFTER INSERT trigger. However, you cannot modify a column value in an AFTER UPDATE trigger...
In addition, you cannot perform an update on the table used in your AFTER INSERT trigger as (http://dev.mysql.com/doc/refman/5.0/en/stored-program-restrictions.html):
Within a stored function or trigger, it is not permitted to modify a
table that is already being used (for reading or writing) by the
statement that invoked the function or trigger.
Here the only reasonable solution would be to create a stored procedure to update the table, adjusting the relevant columns in a transaction to "emulate" you atomic insert statement.
That being said, in your particular case, the key column is redundant as that column is just the concatenation of two other columns of the same row.
Given its name, aren't you looking for a way to create a compound key instead? Something like that:
ALTER TABLE tbl ADD UNIQUE KEY (countryCode, id);
I would like to use "insert on duplicate key update" in a query to either insert a new row if it does not exist or update a row if it does. What I can not seem to figure out is how to use this if I do not have the unique id (because the row has not yet been created, and this ID will be autoincremented upon insert)
insert into foodchoices (unique,notunique) values (Idonthavethis,'test')
on duplicate key update notunique = 'stuff';
Now, in this example above, where it says "Idonthavethis", I do not have any unique value for this field, because it has not yet been inserted as a row. However, I do expect that this inserts somehow, I just dont know how. I expect it to act like this:
insert into foodchoices (notunique) values ('test')
BUT, if it is a field that does already exist, I WILL have that unique value. Is there some form of wildcard or something I can use for when I do not have the unique value?
I believe the answer to this is addressed in the MySQL docs:
If a table contains an AUTO_INCREMENT column and INSERT ... UPDATE inserts a row, the LAST_INSERT_ID() function returns the AUTO_INCREMENT value. If the statement updates a row instead, LAST_INSERT_ID() is not meaningful. However, you can work around this by using LAST_INSERT_ID(expr). Suppose that id is the AUTO_INCREMENT column. To make LAST_INSERT_ID() meaningful for updates, insert rows as follows:
INSERT INTO table (a,b,c) VALUES (1,2,3)
ON DUPLICATE KEY UPDATE id=LAST_INSERT_ID(id), c=3;
I think what you just might try to do is select the row with the value you have (if exists then update) otherwise insert. It's just one more sentence.
I just don't see how can you compare an existing value that you just don't have (the generated ID). Plus, if the ID is DB seeded how it'll be duplicated?
May be you need to alter your table structure for adding any constraint to "notunique" column. So you can:
insert into foodchoices (notunique) values ('test') on duplicate key update columntostoreyouruniqueid = unique;
if notunique has no constaint then it mean that you will have uniqueid as set. So it has to double query.