I have: something like
UPDATE table
SET field = (SELECT field FROM another_table WHERE id = #id);
Problem: SELECT field FROM another_table WHERE id = #id subquery can return one field or EMPTY SET.
Question: How to handle situation when subquery returns empty set?
Updated:
UPDATE table t
SET field = IF((SELECT field FROM another_table WHERE id = #id) IS NOT NULL, -- select field
(SELECT field FROM another_table WHERE id = #id), -- Problem #1: select field AGAIN!
(SELECT field FROM table WHERE id = t.id) -- Problem #2: try to not change value, so select the current field value!!
);
If function can be useful:
UPDATE table
SET field = if((SELECT field FROM another_table WHERE id = #id) IS NULL,true,false);
You can add the conditional:
WHERE (SELECT COUNT(*) FROM another_table WHERE id = #id) > 0
This will make sure that at least one row exists in another_table with the id. See my SQL Fiddle as an example.
Note: this may not be the most efficient because it does a count on another_table, and if it is greater than 1 it will do another SELECT (two sub-queries). Instead, you can do an INNER JOIN:
UPDATE table
INNER JOIN another_table ON table.id=another_table.id
SET table.field = another_table.field
WHERE another_table.id = #id;
See this SQL Fiddle. The reason why I saved this as a second option, is not all SQL languages can UPDATE with joins (MySQL can). Also, you need some way to relate the tables..in this case I said that the table.id we are updating is equal to another_table.id we are taking the data from.
NOTE The UPDATE statement will modify EVERY row in table and assign the same value to every row; that seems a little unusual.
To answer your question:
If you want to handle the "empty set" by not updating any rows in table, then one way to do this is with a JOIN to an inline view:
UPDATE table t
CROSS
JOIN (SELECT a.field
FROM another_table a
WHERE a.id = #id
LIMIT 1
) s
SET t.field = s.field
Note that if the inline view query (aliased as s) return an "empty set", then no rows in table will be updated, because the JOIN operation will also return an "empty set", meaning there are zero rows to be updated.
Related
I have 2 tables; alternate identifier table and a master table
the above image represents the column from alternate identifiers table
and master table have these columns
Now, First I fetch company_group_id from alternate identifier table where bank_entity_id='somevalue' and id_value = 'somevalue';
if this query return nothing than end. if this query return company_group_id than I will check that company_group_id in master table where company_group_id = selected company group id.
after getting this if its present in the table than i will check hashcode if it presents in the master table.
separate queries are like this:
select company_group_id from aes_batch.aes_company_group_alternate_identifiers
WHERE ID_VALUE = '525' and BANK_ENTITY_ID='UOBS';
select company_group_id, hashcode from aes_batch.aes_company_group_master where company_group_id = 'value from the last query' and hashcode="value";
I want to combine these queries to get same result.
this is what I tried but failed.
SELECT t1.company_group_id
FROM aes_batch.aes_company_group_alternate_identifiers t1
LEFT JOIN aes_batch.aes_company_group_master t2
ON t1.company_group_id = t2.company_group_id
WHERE t2.company_group_id IS NULL;
SELECT *
FROM aes_batch.aes_company_group_master B
WHERE NOT EXISTS (SELECT 1
FROM aes_batch.aes_company_group_alternate_identifiers A
WHERE B.company_group_id = A.company_group_id);
Select a.company_group_id, b.company_group_id,
b.hashcode, a.id_value
from aes_batch.aes_company_group_alternate_identifiers a
LEFT JOIN aes_batch.aes_company_group_master b
ON b.company_group_id = a.company_group_id
WHERE a.id_value in ('524','525')
and a.bank_entity_id='UOBS';
can someone help me in this?
Is it possible to create a trigger that conditionally updates a column with a random value from another tables column.
Previously I received help to create a trigger that updates a column with a random value from another tables column: MySQL: Trigger Update with random value from another tables column. I’m trying now to make it conditionally based on another columns value.
If the users.selection column = ‘fruits’ then random select from fruits.
If the users.selection column = ‘animals’ then random from animals.
If neither ‘fruits’ nor ‘animals’ don’t update.
Here is a db-fiddle: https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=6bc76ed2c104dad0e27dd35b1da112a7
Major thanks to #Akina for getting me this far! Lots to learn.
Update (May 29th):
I still can’t figure it out. I thought maybe I would need a SELECT with IF statement first to return the selection column value but that didn’t seem to work. Basically I have tried a lot of different combinations using these examples below as templates. None of them seem to bring my closer.
Anyone have any ideas?
Examples:
SELECT T1.ID, IFNULL(T1.name, T2.name) AS name
FROM firsttable T1
LEFT JOIN secondtable T2
ON T1.T2_id = T2.id
SET final_price= CASE
WHEN currency=1 THEN 0.81*final_price
ELSE final_price
END
SET col = (
SELECT other_col
FROM other_table
WHERE other_table.table_id = table.id
);
SELECT book_name,isbn_no,
IF((SELECT COUNT(*) FROM book_mast WHERE pub_lang='English')>
(SELECT COUNT(*) FROM book_mast WHERE pub_lang<>'English'),
(CONCAT("Pages: ",no_page)),(CONCAT("Price: ",book_price)))
AS "Page / Price"
FROM book_mast;
I think you need to conditionally define what does what, if selection is fruit, then do something. else if selection is animals, then do another thing.
e.g:
CREATE TRIGGER trigger_test
BEFORE UPDATE
ON users
FOR EACH ROW
BEGIN
IF (NEW.selection = 'fruits') THEN
SET NEW.random = ( SELECT fruits
FROM list
ORDER BY RAND() LIMIT 1 );
ELSEIF (NEW.selection = 'animals') THEN
SET NEW.random = ( SELECT animals
FROM list
ORDER BY RAND() LIMIT 1 );
END IF;
END;
Definitely a basic question, but I couldn't find an example.
I'm writing a procedure which merges two rows into the good row. It moves all child rows' ids to being the correct one, replaces all NULL values with available values in the row being removed before finally deleting the 'bad' row.
What I have so far is this:
CREATE DEFINER=`danielv`#`%`
PROCEDURE `emp_merge`(IN `#core_emp_id` int, IN `#bad_emp_id` int)
BEGIN
UPDATE claim SET employee_id = #core_emp_id
WHERE employee_id = #bad_emp_id;
WITH bad_employee_values AS (
SELECT * FROM employee WHERE employee_id = #bad_emp_id
)
UPDATE employee SET
employee.employment_date = COALESCE(employee.employment_date, bad_employee_values.employment_date),
WHERE employee_id = #core_emp_id;
DELETE FROM employee WHERE employee_id = #bad_emp_id;
END
However, I'm getting non-descript error messages and I'm not sure why. I suspect there's an issue with how I'm handling my CTE and coalesce function, but I'm not sure where the gap in my understanding is.
In this statement :
WITH bad_employee_values AS (SELECT * FROM employee WHERE employee_id = #bad_emp_id)
UPDATE employee SET
employee.employment_date = COALESCE(employee.employment_date, bad_employee_values.employment_date),
WHERE employee_id = #core_emp_id;
You are defining CTE bad_employee_values but you are not using it in the UPDATE part of the query, hence you cannot access its columns : for MySQL, bad_employee_values.employment_date is unknown.
It looks like you could simply avoid a CTE here. You could just self-join the table, like so :
UPDATE employee e_core
INNER JOIN employee e_bad ON e_bad.employee_id = #bad_emp_id
SET e_core.employment_date = e_bad.employment_date,
WHERE employee_id = #core_emp_id AND e_core.employment_date IS NULL
This query will simply select the record identified by #core_emp_id, join it with the corresponding "bad" record, and copy the value of employment_date. The second condition in the WHERE clause prevents records whose employment_date is not null from being selected.
I'm looking for a simple way to do an update on a table only if there is no other columns present in that same table with the same value I'm trying to update, ideally in a single query. So far I'm getting an error You specify target table 't1' for update in FROM clause. Here is what I tried in a few variations so far (still unable to get working):
UPDATE emailQueue AS t1
SET
t1.lockedOn = 1470053240
WHERE
(SELECT
COUNT(*)
FROM
emailQueue AS t2
WHERE
t2.lockedOn = 1470053240) = 0
AND t1.lockedOn IS NULL
In MySQL, you need to use a join. In this case, a left join is in order:
UPDATE emailQueue eq LEFT JOIN
emailQueue eq2
ON eq2.lockedOn = 1470053240
SET eq.lockedOn = 1470053240
WHERE eq.lockedOn IS NULL AND
eq2.lockedOn IS NULL;
I have 2 tables: tbl_taxclasses, tbl_taxclasses_regions
This is a one to many relationship, where the main record ID is classid.
I have a column inside the first table called regionscount
So, I create a Tax Class, in table 1. Then I add regions/states in table 2, assigning the classid to each region.
I perform a SELECT statement to count the regions with that same classid, and then I perform an UPDATE statement on tbl_taxclasses with that number. I update the regionscount column.
This means I'm writing 2 queries. Which is fine, but I was wondering if there was a way to do a SELECT statement inside the UPDATE statement, like this:
UPDATE `tbl_taxclasses` SET `regionscount` = [SELECT COUNT(regionsid) FROM `tbl_taxclasses_regions` WHERE classid = 1] WHERE classid = 1
I'm reaching here, since I'm not sure how robust MySQL is, but I do have the latest version, as of today. (5.5.15)
You could use a non-correlated subquery to do the work for you:
UPDATE
tbl_taxclasses c
INNER JOIN (
SELECT
COUNT(regionsid) AS n
FROM
tbl_taxclasses_regions
GROUP BY
classid
) r USING(classid)
SET
c.regionscount = r.n
WHERE
c.classid = 1
Turns out I was actually guessing right.
This works:
UPDATE `tbl_taxclasses`
SET `regionscount` = (
SELECT COUNT(regionsid) AS `num`
FROM `tbl_taxclasses_regions`
WHERE classid = 1)
WHERE classid = 1 LIMIT 1
I just needed to replace my brackets [] with parenthesis ().