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.
Related
There's 2 unconnected tables, with no common IDs
+---------------------+
| names |
+------+--------------+
| name | lastN |
+-------------+-------+
| Anthony | monta |
| Ryo | shizu |
+------+--------------+
+----------------------+
| nicknames |
+------+---------------+
| nickname |
+------+---------------+
| miso_hungry |
+------+---------------+
I'm trying to run a select query on both tables and currently doing something like:
SELECT names.name, nicknames.nickname
FROM names, nicknames
WHERE names.name="miso_hungry" OR nicknames.nickname="miso_hungry"
I'm getting back a weird results with repeating identical rows, which doesn't make sense.
For example if I search for miso_hungry with the query above it will return every row of "names" table for some reason and append the correct rows from the "nicknames" table..
Attaching a screenshot of the results
Above should show "NULL" under name column, since "miso_hungry" is not found in that column and I'm not sure why it prints every row of the "names" table also.
You can use UNION Clause
Each SELECT statement within UNION must have the same number of columns
The columns must also have similar data types
The columns in each SELECT statement must also be in the same order
So we need to made them satisfy above condition. We can use Aliasing to do this.
SELECT name,(SELECT NULL) as nickname FROM names WHERE name = "miso_hungry"
UNION
SELECT (SELECT NULL) as name, nickname FROM nicknames WHERE nickname = "miso_hungry"
Edited
If you want to get the match count from both table use query like below :
SELECT SUM(count) as count FROM (
SELECT count(*) as count FROM names WHERE name = "miso_hungry"
UNION ALL
SELECT count(*) as count FROM nicknames WHERE nickname = "miso_hungry"
) both_table
The order of execution in your statement is from,where,select. With and implicit join you get a cartesian product which given
use sandbox;
create table n(name varchar(20));
create table nn(nickname varchar(20));
insert into n values('Antony'),('Ryo');
insert into nn values('miso');
results in
MariaDB [sandbox]> SELECT n.name, nn.nickname
-> FROM n, nn;
+--------+----------+
| name | nickname |
+--------+----------+
| Antony | miso |
| Ryo | miso |
+--------+----------+
2 rows in set (0.00 sec)
The where clause is then applied - which yields the same result.
What does it mean?
SELECT * from users where password = ''*'';
if I check this in mysql workbench I get only one line, although I have lot of users in table.
What exactly does this select?
Interesting question. Let's see what ''*'' does.
mysql> select ''*'';
+-------+
| ''*'' |
+-------+
| 0 |
+-------+
Let's create some users:
mysql> select * from users;
+------+-------+
| id | name |
+------+-------+
| 1 | joe |
| 2 | moe |
| 3 | shmoe |
| 4 | 4four |
+------+-------+
And test our query:
mysql> select * from users where name = ''*'';
+------+-------+
| id | name |
+------+-------+
| 1 | joe |
| 2 | moe |
| 3 | shmoe |
+------+-------+
Interestingly enough, user 4 was not selected! But let's try this way:
mysql> select * from users where name = 4;
+------+-------+
| id | name |
+------+-------+
| 4 | 4four |
+------+-------+
So, what can we deduct from this?
''*'' somehow means 0 (I am not that fluent in mysql string operators, so let's take it as a fact);
MySQL, apparently, does type conversions in this case. So if you query a varchar column against an integer, it tries to convert those strings to ints and see if it's a match;
You have only one row whose password begins with 0 or non-digit.
You can always use an expression in SQL. Like SELECT 5-4 AS one and get 1. So you can tell that here is an expression.
MySQL is a loosely typed language, so it can multiply strings. Casting them to numbers. And get you zero as a result of '' * ''
When comparing a string with a number, MySQL casts both to a number. So 0 = 'name' condition will get you true
The ''*'' is a multiplication: its two arguments (empty strings) are converted to numericals (i.e. 0) and the result is 0. Then the left side of the equation is also converted to a number, which will sometimes be zero (when the password cannot be evaluated to a non-zero number), sometimes not.
It is a bit obscure, and you could ask yourself whether this was intended in your case or an accidental behaviour, while the actual intention was to test for '*'. A user with bad intentions might have entered '*' as a password hoping you were not protected against SQL injection in order to get into the system without a valid password.
I have an SQL query:
select DISTINCT shortname_chn from dim_channel;
The query returns me data for example:
| shortname_chn (VARCHAR) |
|__________________________|
| MTV |
| National Geographic|
| Discovery |
| ARY News |
How can I manipulate the SQL query so that I can add an additional row to the returned rows.
Following is the result I wish to get after running some query:
| shortname_chn (VARCHAR) |
|__________________________|
| MTV |
| National Geographic|
| Discovery |
| ARY News |
| ALL |
Where the last row "ALL" is user defined, not present in the database.
In the above mentioned regard, I researched and came across this question : How to add a user defined column with a single value to a SQL query but it targets the problem of adding a whole new column.
select DISTINCT shortname_chn from dim_channel
UNION
SELECT 'ALL'
You can simply do something like this by UNIONing with a query that returns your fake row, e.g.:
SELECT DISTINCT
shortname_chn
FROM dim_channel
UNION ALL
SELECT 'ALL' AS shortname_chn
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.
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.