I'm just getting into SQL and need a hand with some of the basics.
I'm trying to write a query that will allow me to find values between range in one column of a table and replace all values in that range with a single value that I would specify with an UPDATE or REPLACE function.
I've gotten as far as
SELECT *
FROM (Table_A)
WHERE Column BETWEEN #a and #b
ORDER by Column desc
but I get back 0 results even though I know there are tons of values between #a and #b.
Select with BETWEEN
SELECT * FROM Table_A WHERE column BETWEEN 10 AND 30
Update
update Table_A set column = 'NEW_VALUE' where column BETWEEN 10 AND 30
Replace
// string hallo2 -> will be change to hallo3
update Table_A set column = replace(column, "2", "3") where column BETWEEN 10 AND 30
MySQL/SQLite
I want to insert a randomly generated number (of 9 positions) into multiple rows BUT they need to be the same for all rows matched in the query.
update products set tag_seed=( SELECT ABS(RANDOM() % 999999999) ) where [...];
Partialy works... Each row will have a different random number. I need them to be same.
This is logical since you will generate a new random for every update query. The easiest solution is to generate a random integer, store it in a local variable and use that variable in your queries:
SET #rand := (SELECT ABS(RAND() * 1000000000));
update products set tag_seed=#rand where [...];
First we start with empty table
rows = 0
Second we insert random rows let say 3400
rows = 3400
For the third time i count how many rows are in the table, then insert the new rows and after that delete rows <= from the count.
This logic only work for the first time. If that repeat the count will always be 3400 but the id will increase so it will not delete the rows
I cant use last inserted ID since the rows are random and I dont how many it will load.
// Update
"SELECT count(*) from table" - the total count so far
"INSERT INTO tab_videos_watched (id , name) values (id , name)" - this is random can be 3400 or 5060 or 1200
"DELETE FROM table WHERE idtable <= $table_count"
If id is auto incremented, then you should use like:
select max(id) from my_table;
Read this maxId into a variable and then use when issued a delete query like:
delete from my_table where id <= ?;
Replace query parameter with the last found maxId value.
Alternatively you can define a column last_inserted as datetime type.
Before next insertions, select it into a local variable.
select max(last_inserted) as 'last_inserted' from my_table;
And after insertions are made, use the last_inserted to delete records.
delete from my_table where last_inserted <= ?;
Replace query parameter with the last found last_inserted value.
How can I store only 10 rows in a MySQL table? The older rows should be deleted when a new row is added but only once the table has 10 rows.
Please help me
You could achieve this with an after insert trigger, delete the row where it is min date. e.g. DELETE FROM myTable WHERE myTimestamp = (SELECT MIN(myTimestamp) FROM myTable) but that could in theory delete multiple rows, depending on the granularity of your updates.
You could have an incrementing sequence, and always just delete the min of that sequence.
The question is why you'd want to do this though? It's a slightly unusual requirement.
A basic example (not validated/executed, I don't have mySQL on this particular machine) would look something like.
CREATE TRIGGER CycleOldPasswords AFTER INSERT ON UserPasswords FOR EACH ROW
set #mycount = SELECT COUNT(*) FROM UserPasswords up where up.UserId = NEW.UserId;
if myCount >= 10 THEN
DELETE FROM UserPasswords up where up.Timestamp = (SELECT min(upa Timestamp) FROM UserPasswords upa WHERE NEW.UserId = upa.UserId) AND NEW.UserId = up.UserId;
END
END;
You can retrieve the last inserted id when your first row is inserted, and store it in a variable. When 10 rows are inserted, delete the row having id < id of the first inserted record. Please try it.
first of all insert all values using your insert query
and then run this query
delete from table_name where (cond) order by id desc limit 10
must specify an id or time in one column
First, here's the concise summary of the question:
Is it possible to run an INSERT statement conditionally?
Something akin to this:
IF(expression) INSERT...
Now, I know I can do this with a stored procedure.
My question is: can I do this in my query?
Now, why would I want to do that?
Let's assume we have the following 2 tables:
products: id, qty_on_hand
orders: id, product_id, qty
Now, let's say an order for 20 Voodoo Dolls (product id 2) comes in.
We first check if there's enough Quantity On Hand:
SELECT IF(
( SELECT SUM(qty) FROM orders WHERE product_id = 2 ) + 20
<=
( SELECT qty_on_hand FROM products WHERE id = 2)
, 'true', 'false');
Then, if it evaluates to true, we run an INSERT query.
So far so good.
However, there's a problem with concurrency.
If 2 orders come in at the exact same time, they might both read the quantity-on-hand before any one of them has entered the order.
They'll then both place the order, thus exceeding the qty_on_hand.
So, back to the root of the question:
Is it possible to run an INSERT statement conditionally, so that we can combine both these queries into one?
I searched around a lot, and the only type of conditional INSERT statement that I could find was ON DUPLICATE KEY, which obviously does not apply here.
INSERT INTO TABLE
SELECT value_for_column1, value_for_column2, ...
FROM wherever
WHERE your_special_condition
If no rows are returned from the select (because your special condition is false) no insert happens.
Using your schema from question (assuming your id column is auto_increment):
insert into orders (product_id, qty)
select 2, 20
where (SELECT qty_on_hand FROM products WHERE id = 2) > 20;
This will insert no rows if there's not enough stock on hand, otherwise it will create the order row.
Nice idea btw!
Try:
INSERT INTO orders(product_id, qty)
SELECT 2, 20 FROM products WHERE id = 2 AND qty_on_hand >= 20
If a product with id equal to 2 exists and the qty_on_hand is greater or equal to 20 for this product, then an insert will occur with the values product_id = 2, and qty = 20. Otherwise, no insert will occur.
Note: If your product ids are note unique, you might want to add a LIMIT clause at the end of the SELECT statement.
Not sure about concurrency, you'll need to read up on locking in mysql, but this will let you be sure that you only take 20 items if 20 items are available:
update products
set qty_on_hand = qty_on_hand - 20
where qty_on_hand >= 20
and id=2
You can then check how many rows were affected. If none were affected, you did not have enough stock. If 1 row was affected, you have effectively consumed the stock.
You're probably solving the problem the wrong way.
If you're afraid two read-operations will occur at the same time and thus one will work with stale data, the solution is to use locks or transactions.
Have the query do this:
lock table for read
read table
update table
release lock
I wanted to insert into a table using values so I found this solution to insert the values using the IF condition
DELIMITER $$
CREATE PROCEDURE insertIssue()
BEGIN
IF (1 NOT IN (select I.issue_number from issue as I where I.series_id = 1)) THEN
INSERT IGNORE INTO issue ( issue_number, month_published, year_published, series_id, mcs_issue_id) VALUES (1, 1, 1990, 1, 1);
END IF;
END$$
DELIMITER ;
If you later on want to call the procedure it's as simple as
CALL insertIssue()
You can find more information about PROCEDURES and if conditions in this site