Table name : Students.
The Table i have:
mysql> SELECT * from Students;
+-----------+-------------+-------+
| Rollno | Name | Marks |
+-----------+-------------+-------+
| 251602122 | Sumit Tyagi | 70 |
| 251602121 | parveen | 90 |
+-----------+-------------+-------+
Following query returns the following result even 8 is not a attribute.
mysql> select 8 from Students;
+---+
| 8 |
+---+
| 8 |
| 8 |
+---+
Similarly
mysql> SELECT 'some_string' from Students;
+-------------+
| some_string |
+-------------+
| some_string |
| some_string |
I just want to know why this happens.
The query returns one line for every record in your table.
But you don't select data from those record. You just select the number 8 for each line. And this gets returned.
Select statement looks for column name in a table. You can make sure SQL look for a column name in a table by using TableName.ColumnName.
In the example you wrote, you are asking for a constant or hardcoded value 8/some_string to be returned from the table which is not the column name. So it will return the hardcoded or constant value you asked for, the number of times equal to number of rows in your table.
If you want to make sure it look for the column name, use the syntax I mentioned as TableName.ColumnName. You can also provide an alias for your table. So in the example above, if you use the syntax as
SELECT Students.8 from Students;
or
SELECT s.8 FROM Students s;
It will look for column name as 8 instead of constant or hardcoded value 8.
If I am not wrong, it is a best practice to use TableName.ColumnName or alias.ColumnName while writing queries as it checks for column name in that particular table.
I have to make a SQL query in Mysql to search a string list (for ex: 1,2,3) in a columns (for ex: list_id), which also have string value list (1,2,3).
For more detail, my_table is
+-----------+----------+
| id | list_id |
+-----------+----------+
| 1 | 29 |
| 2 | 30 |
| 3 | 31 |
| 4 | 4,5,6,7 |
| 5 | 8,9,10,11|
| 6 | 4,5,8,9 |
| 7 | 1,2,3,6 |
+-----------+----------+
The search value is 1,5,8 and I need get the rows have list_id have 1 or 5 or 8 in it's list. Therefore, the result wil be:
+-----------+----------+
| id | list_id |
+-----------+----------+
| 4 | 4,5,6,7 |
| 5 | 8,9,10,11|
| 6 | 4,5,8,9 |
| 7 | 1,2,3,6 |
+-----------+----------+
My query string is:
SELECT * FROM my_table
WHERE list_id LIKE '%,1,%'
OR list_id LIKE '1,%'
OR list_id LIKE '%,1'
OR list_id LIKE '%,5,%'
OR list_id LIKE '5,%'
OR list_id LIKE '%,5'
OR list_id LIKE '%,8,%'
OR list_id LIKE '8,%'
OR list_id LIKE '%,8'
It is match correct what I want. However, the length of query is in proportion to length of list.
Does REGEXP is better than LIKE in this circumstance?
Does anyone have experience to make another solution better?
You may try to concatenate commas to your field (or use SET in MySQL or make a better database structure - in which you join on tables in which the related data is stored).
SELECT * FROM yourtable WHERE CONCAT(',', fieldname, ',') like '%,1,%';
Yes, regular expressions will work for this. Here is what you can do:
SELECT * FROM junk
WHERE CONCAT(',', list_id, ',') REGEXP CONCAT(',(', REPLACE('1,3,8',',','|'), '),');
Results:
ID | LIST_ID
5 | 8,9,10,11
6 | 4,5,8,9
7 | 1,2,3,6
Please see SQL Fiddle demo here.
We turn the query list 1,3,8 into an alternating group 1|3|8. You might be able to do this in your application code to avoid using the REPLACE() function above.
UPDATE Apologies, I mistakenly used 1,3,8 as the query parameter instead of 1,5,8. But it should still work.
I am going to strongly suggest that you change the design of the database (I am assuming you have some control or influence over it).
You should make the id column non-unique and then the list_id column should contain a single value. You can then search as follows:
SELECT id WHERE list_id IN (1,5,8)
If it is a big table and there are a lot of list_id values, put an index on the list_id column.
If you need the output in a comma-separated list, then you will need to use an aggregating concatenation function with GROUP BY (e.g., GROUP_CONCAT() in MySQL).
If you cannot change the design of the schema, then use one of the other suggestions here.
Hello everyone,
I'm just wondering if the below is possible.
Let's say i have this table
+---------+-------+
| item_id | price |
+---------+-------+
| 14 | 1500 |
| 74 | 1500 |
| 78 | 2500 |
+---------+-------+
And I'd like to select all item_id within this array (14,78,14)....
I thought I would be able to get the sum of all prices with this select:
mysql> select SUM(price) from items_db where item_id in (14,78,14);
+-------+
| price |
+-------+
| 4000 |
+-------+
1 row in set (0.00 sec)
But obviously it just skipped the price for the duplicate item_id.
So is there any way I could make this work?
Thanks in advance,
Alex
You can't. You need to split the values that you receive, insert them on a temporary table and then do a join with the table that contains the values you wish to sum.
Wouldn't it be easy to do the select and then postprocess it with whatever language are you comfortable with?
ok got it... if you list duplicates in the IN(...) it wont work.
At each row the database engine checks wether the rows matchs the IN (... ) list.
So it is no going to read a row twice in such a statement
One way to read a given row several times is through a join... If you want to do such a sum() through SQL you have to join an intermediate (temporary) table. something like:
create temporary table itemlist (item_id);
insert into itemlist values (14),(78),(14);
select sum(price) from items_db inner join itemlist using (item_id);
+------------+
| sum(price) |
+------------+
| 5500 |
+------------+
I have a query that retrieves some data, among those data I have some that are returned with a value like 0. I would like the query to NOT return the columns when that's the case.
How can we do such a thing?
Regards,
MEM
select <column_name> from <table_name> where <column_name> <> 0.0
Here is all the data in a sample database. Notice how there are 3 rows with one having a zero value for the num column.
mysql> select * from test_tbl;
+------+----------+
| num | some_str |
+------+----------+
| 0 | matt |
| 2 | todd |
| 3 | Paul |
+------+----------+
3 rows in set (0.00 sec)
Now lets use the where clause to specify the rows we want to ignore (it's a little bit of reverse logic because we are actually specifying what rows we want).
mysql> select * from test_tbl where num <> 0.0;
+------+----------+
| num | some_str |
+------+----------+
| 2 | todd |
| 3 | Paul |
+------+----------+
2 rows in set (0.00 sec)
Note: This will only work without getting messy if 0 is the only value you are worried about. A better way would be to allow nulls in your column and then you can check to see if they are non-null in the where clause.
I've got a table in MySQL that looks roughly like:
value | count
-------------
Fred | 7
FRED | 1
Roger | 3
roger | 1
That is, it was created with string ops outside of MySQL, so the values are case- and trailing-whitespace-sensitive.
I want it to look like:
value | count
-------------
Fred | 8
Roger | 4
That is, managed by MySQL, with value a primary key. It's not important which one (of "Fred" or "FRED") is kept.
I know how to do this in code. I also know how to generate a list of problem values (with a self-join). But I'd like to come up with a SQL update/delete to migrate my table, and I can't think of anything.
If I knew that no pair of records had variants of one value, with the same count (like ("Fred",4) and ("FRED",4)), then I think I can do it with a self-join to copy the counts, and then an update to remove the zeros. But I have no such guarantee.
Is there something simple I'm missing, or is this one of those cases where you just write a short function outside of the database?
Thanks!
As an example of how to obtain the results you are looking for with a SQL query alone:
SELECT UPPER(value) AS name, SUM(count) AS qty FROM table GROUP BY name;
If you make a new table to hold the correct values, you INSERT the above query to populate the new table as so:
INSERT INTO newtable (SELECT UPPER(value) AS name, SUM(count) AS qty FROM table GROUP BY name);
Strangely, MySQL seems to do this for you. I just tested this in MySQL 5.1.47:
create table c (value varchar(10), count int);
insert into c values ('Fred',7), ('FRED',1), ('Roger',3), ('roger',1);
select * from c;
+-------+-------+
| value | count |
+-------+-------+
| Fred | 7 |
| FRED | 1 |
| Roger | 3 |
| roger | 1 |
+-------+-------+
select value, sum(count) from c group by value;
+-------+------------+
| value | sum(count) |
+-------+------------+
| Fred | 8 |
| Roger | 4 |
+-------+------------+
I was surprised to see MySQL transform the strings like that, and I'm not sure I can explain why it did that. I was expecting to have to get four distinct rows, and to have to use some string functions to map the values to a canonical form.