How can I update a field in mysql based on the previous value?
Lets say that count equals 3.
UPDATE MY_TABLE SET count = count-1
Will that work? so the new value of count would be 4
Did you mean for the row or the table? If you mean on insert or update for a row you can use triggers.
I gave it a try and it worked, so perhaps you need to show us the CREATE statement for the table. Test results:
mysql> create table aint ( count int not null default 0 );
Query OK, 0 rows affected (0.25 sec)
mysql> insert into aint VALUES(1);
Query OK, 1 row affected (0.06 sec)
mysql> select * from aint;
+-------+
| count |
+-------+
| 1 |
+-------+
1 row in set (0.00 sec)
mysql> update aint set count = count-1;
Query OK, 1 row affected (0.06 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from aint;
+-------+
| count |
+-------+
| 0 |
+-------+
1 row in set (0.00 sec)
mysql> update aint set count = count-1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from aint;
+-------+
| count |
+-------+
| -1 |
+-------+
1 row in set (0.00 sec)
Related
When I use CREATE TABLE tbl_name I can only specify a table in database I currently use.
Is there any way to CREATE TABLE tbl_name in database1 without a prior USE database1?
Try:
mysql> CREATE DATABASE `test_1`;
Query OK, 1 row affected (0.00 sec)
mysql> CREATE DATABASE `test_2`;
Query OK, 1 row affected (0.00 sec)
mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| NULL |
+------------+
1 row in set (0.00 sec)
mysql> CREATE TABLE `test_1`.`table_1` (`column_1` BOOL);
Query OK, 0 rows affected (0.00 sec)
mysql> CREATE TABLE `test_2`.`table_2` (`column_1` BOOL);
Query OK, 0 rows affected (0.01 sec)
mysql> USE `test_1`;
Database changed
mysql> SHOW TABLES;
+------------------+
| Tables_in_test_1 |
+------------------+
| table_1 |
+------------------+
1 row in set (0.00 sec)
mysql> USE `test_2`;
Database changed
mysql> SHOW TABLES;
+------------------+
| Tables_in_test_2 |
+------------------+
| table_2 |
+------------------+
1 row in set (0.00 sec)
i have inserted a column in mysql table with backslash(\)like user\12345.But when i see in the table values i am not seeing the forward slash(\). I am just seeing user12345.
When i query the just inserted row with backslash(\) with the below possible ways, i am always getting the zero records.
select * from users where user='user12345'
select * from users where user='user\12345'
select * from users where user='user\\12345'
when i copy paste the column value into notepad+ i could see there is a carriage retun in the end of every column values. user12345CRLF, as of now i can't update/remove the carriage return. but i want to fetch the records with carriage return. how can i do this?
I don't want to use like query, and i want to query for the exact username. how can i do this?
Backslash character \ is used for escape sequence. when you insert value like 'user\12345' then by default mysql server takes it as '\1' as escape character but it is not any special character so it discard the \ from the string.
Hitesh> update test set fname='user\344';
Query OK, 2 rows affected (0.06 sec)
Rows matched: 2 Changed: 2 Warnings: 0
Hitesh> select fname from test;
+---------+
| fname |
+---------+
| user344 |
| user344 |
+---------+
2 rows in set (0.00 sec)
If you want to store \ in string then you should use \
Hitesh> update test set fname='user\\344';
Query OK, 2 rows affected (0.03 sec)
Rows matched: 2 Changed: 2 Warnings: 0
Hitesh> select fname from test;
+----------+
| fname |
+----------+
| user\344 |
| user\344 |
+----------+
2 rows in set (0.00 sec)
if you don't want to set \ as escape character then you can enable the sql mode NO_BACKSLASH_ESCAPES.
Hitesh> SET sql_mode = 'NO_BACKSLASH_ESCAPES';
Query OK, 0 rows affected (0.00 sec)
Hitesh> update test set fname='user\344';
Query OK, 0 rows affected (0.00 sec)
Rows matched: 2 Changed: 0 Warnings: 0
Hitesh> select fname from test;
+----------+
| fname |
+----------+
| user\344 |
| user\344 |
+----------+
2 rows in set (0.00 sec)
for case of carriage return to end of column, in query you can find the string which ends with \r in where clause and select column you can skip the last character which is \r
Hitesh> set ##sql_mode='STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION';
Query OK, 0 rows affected (0.00 sec)
Hitesh> update test set fname='user344\r';
Query OK, 2 rows affected (0.17 sec)
Rows matched: 2 Changed: 2 Warnings: 0
Hitesh> select left( fname, CHAR_LENGTH(fname)-1) as fname from test where fname like '%\r';
+---------+
| fname |
+---------+
| user344 |
| user344 |
+---------+
2 rows in set (0.00 sec)
Thanks a ton #Hitesh,#Barmar again. I figured it out the query for exact match search. select * from users where user='user12345\r'
I think I read that the delete trigger doesn't know what data was deleted and loops over the whole table applying the trigger. Is that true?
Does that mean that the before delete loops over the whole table before the data is deleted and after delete loops over the whole table after the delete occurs?
Is there no way to loop over just the deleted records? So If 10 records are deleted loop over them?
DELIMITER $$
DROP TRIGGER `before_delete_jecki_triggername`$$
CREATE TRIGGER before_delete_triggername
BEFORE DELETE ON table
FOR EACH ROW
BEGIN
/*do stuff*/
END$$
DELIMITER ;
Thanks,
Mat
I think it was due to a confusion with FOR EACH ROW statement.
It is only for "matched records for the statement issued before trigger is invoked."
If there exists N number of records in a table and matches records for where id=x,
assuming x causes a result of less than N records, say N-5, then
FOR EACH ROW causes a loop for N-5 times only.
UPDATE:
A sample test run on the rows affected due to FOR EACH ROW statement is shown below.
mysql> -- create a test table
mysql> drop table if exists tbl; create table tbl ( i int, v varchar(10) );
Query OK, 0 rows affected (0.01 sec)
Query OK, 0 rows affected (0.06 sec)
mysql> -- set test data
mysql> insert into tbl values(1,'one'),(2,'two' ),(3,'three'),(10,'ten'),(11,'eleven');
Query OK, 5 rows affected (0.02 sec)
Records: 5 Duplicates: 0 Warnings: 0
mysql> select * from tbl;
+------+--------+
| i | v |
+------+--------+
| 1 | one |
| 2 | two |
| 3 | three |
| 10 | ten |
| 11 | eleven |
+------+--------+
5 rows in set (0.02 sec)
mysql> select count(*) row_count from tbl;
+-----------+
| row_count |
+-----------+
| 5 |
+-----------+
1 row in set (0.00 sec)
mysql>
mysql> -- record loop count of trigger in a table
mysql> drop table if exists rows_affected; create table rows_affected( i int );
Query OK, 0 rows affected (0.02 sec)
Query OK, 0 rows affected (0.05 sec)
mysql> select count(*) 'rows_affected' from rows_affected;
+---------------+
| rows_affected |
+---------------+
| 0 |
+---------------+
1 row in set (0.00 sec)
mysql>
mysql> set #cnt=0;
Query OK, 0 rows affected (0.00 sec)
mysql>
mysql> -- drop trigger if exists trig_bef_del_on_tbl;
mysql> delimiter //
mysql> create trigger trig_bef_del_on_tbl before delete on tbl
-> for each row begin
-> set #cnt = if(#cnt is null, 1, (#cnt+1));
->
-> /* for cross checking save loop count */
-> insert into rows_affected values ( #cnt );
-> end;
-> //
Query OK, 0 rows affected (0.00 sec)
mysql>
mysql> delimiter ;
mysql>
mysql> -- now let us test the delete operation
mysql> delete from tbl where i like '%1%';
Query OK, 3 rows affected (0.02 sec)
mysql>
mysql> -- now let us see what the loop count was
mysql> select #cnt as 'cnt';
+------+
| cnt |
+------+
| 3 |
+------+
1 row in set (0.00 sec)
mysql>
mysql> -- now let us see the table data
mysql> select * from tbl;
+------+-------+
| i | v |
+------+-------+
| 2 | two |
| 3 | three |
+------+-------+
2 rows in set (0.00 sec)
mysql> select count(*) row_count from tbl;
+-----------+
| row_count |
+-----------+
| 2 |
+-----------+
1 row in set (0.00 sec)
mysql> select count(*) 'rows_affected' from rows_affected;
+---------------+
| rows_affected |
+---------------+
| 3 |
+---------------+
1 row in set (0.00 sec)
mysql>
I have a column in a table that contains the path for a file, along the lines of:
/here/here2/something.jpg
I need to change every row to map it to:
/here3/something.jpg
Is there an easy way to do this? Thanks!
You don't need regex for this. Simple string functions can do the trick. You could use a query like this:
update test set path=concat('/here3/',
substring(path, length('/here/here2/') + 1))
where path like '/here/here2/%';
Here is a little test case to prove it works:
mysql> create table test (path varchar(64));
Query OK, 0 rows affected (0.02 sec)
mysql> insert into test (path) values
-> ('/here/here2/something.jpg'),
-> ('/here/here2/something-else.jpg'),
-> ('/here/here2/yet-another-something.jpg');
Query OK, 3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from test;
+---------------------------------------+
| path |
+---------------------------------------+
| /here/here2/something.jpg |
| /here/here2/something-else.jpg |
| /here/here2/yet-another-something.jpg |
+---------------------------------------+
3 rows in set (0.00 sec)
mysql> update test set path=concat('/here3/',
-> substring(path, length('/here/here2/') + 1))
-> where path like '/here/here2/%';
Query OK, 3 rows affected (0.00 sec)
Rows matched: 3 Changed: 3 Warnings: 0
mysql> select * from test;
+----------------------------------+
| path |
+----------------------------------+
| /here3/something.jpg |
| /here3/something-else.jpg |
| /here3/yet-another-something.jpg |
+----------------------------------+
3 rows in set (0.00 sec)
First of all, are you familiar with using PHP? You could write a script to get all of the entries from the database, and then change them using preg_replace(). Then you could use mysql to update the database.
Can there any way to insert a hex value into MYSQL?
I also want to be able to retreive it in hex form.
For example, something like:
INSERT INTO table ( hexTag )
VALUES ( HEX(0x41) );
And if I do this, I want it to put an 'A' into the table
For that particular use case, you can either insert the hex value directly and it will be interpreted as a string, or use HEX() to input and UNHEX() to output
mysql> create table hexTable(pseudoHex varchar(50));
Query OK, 0 rows affected (0.01 sec)
mysql> insert into hexTable values (0x41);
Query OK, 1 row affected (0.00 sec)
mysql> select * from hexTable;
+-----------+
| pseudoHex |
+-----------+
| A |
+-----------+
1 row in set (0.00 sec)
mysql> select HEX(pseudoHex) from hexTable;
+----------------+
| HEX(pseudoHex) |
+----------------+
| 41 |
+----------------+
1 row in set (0.00 sec)
mysql> delete from hexTable;
Query OK, 1 row affected (0.00 sec)
mysql> insert into hexTable values (HEX('A'));
Query OK, 1 row affected (0.00 sec)
mysql> select UNHEX(pseudoHex) from hexTable;
+------------------+
| UNHEX(pseudoHex) |
+------------------+
| A |
+------------------+
1 row in set (0.00 sec)
mysql> select * from hexTable;
+-----------+
| pseudoHex |
+-----------+
| 41 |
+-----------+
1 row in set (0.00 sec)
See these links.