I have a table of 'entries' in a MYSQL database. I have another table that records activity on those entries, with the id of the entry as a foreign key. I want to select from my first table entries that do not appear in the second table.
How can I use SQL to make this happen? Do I have to iterate through both tables and compare every entry with every other entry? Is there an easier way to do this?
ex. I have a table with an entry data column and a user name column. I have another table with an entry id column and a user id column. I want to select from my first table all of the entries which do not appear in the second table with a given user id.
Thanks ahead of time. I have been struggling with this experiment for a while. I imagine I have to join the two tables somehow?
Several ways to achieve this, NOT IN, NOT EXISTS, LEFT JOIN / NULL check. Here's one with NOT EXISTS:
SELECT *
FROM FirstTable T
WHERE NOT EXISTS (
SELECT *
FROM SecondTable T2
WHERE T.Id = T2.Id
)
From what I understand, you want to select all rows where the foreign key doesn't match anything in the other table. This should do the trick:
SELECT *
FROM Data A
RIGHT JOIN Entry B
ON A.ID = B.ID
WHERE A.ID IS NULL
Here's a handy chart that illustrates how to use joins for stuff like this.
You can also use NOT IN, and the mechanics for this one are actually a bit easier to understand.
SELECT *
FROM Data A
WHERE A.ID NOT IN (SELECT ID FROM Entry)
Related
I have a question about merging a table with another preserving an ID on a database (I'm using MySQL). I have 2 tables, the first has and Item ID and a category and subcategory assigned to that ID. The second has a Item ID with all its characteristics like name and other variables. How can I merge those two tables in a way that the ID corresponds to the correct item in the new table (that's the difficult part I think)? Is it possible?
Thank you for all the help!
It's a very basic operation called Inner Join:
Select *
from table1
inner join table2
on table1.itemid = table2.itemid;
EDIT: As OP wants to create a new table with the fields return by above query and insert data into newly created table; following are the query to insert data once its created:
Insert into tablename
Select *
from table1
natural join table2;
Note: Make sure that the order and datatypes of columns in new table and in the result of above select query must be same.
I'm assuming you want to create table from the combined results. See this page for details.
Basically you write and test the SQL query then CREATE TABLE table_name AS sql_query
create table new_item_table
as
select
a.item_id,
a.category,
a.subcategory,
b.item_name,
b.item_char_1,
b.item_char_2
from
item_category a inner join item_char b on a.item_id = b.item_id;
This will Do:
select a.*,b.ItemName,b.ItemChar1,b.ItemChar2 from FirstTable a join select * from SecondTable b on a.ItemId=B.ItemId;
Use left join if some of the records are not there in the second table
I have a TABLE1 like this:
And a TABLE2 like this:
I want to delete entries from table 1 whose endTimestamp is not equal to ANY table 2 entry endTimestamp, with a margin of 1000 time units.
(I know in this example all the entries from table 1 and table 2 have the same timestamp values, so the 5 entries from table 1 should be kept, and any other should be erased if existed)
Since the ids of both tables are not related to each other, I can't perform a JOIN operation as long as I know.
How can I do this?
EDIT: Tried here. Works, but does not work at my server :|
You're looking for :
delete from table1 where endTimestamp not in (Select endTimestamp from table2)
Edit : As pointed out by #user2864740, you can very well use Join here too, even if the ids of both tables are not related to each other.
DELETE FROM table1 INNER JOIN table2 ON table1.endTimestamp = table2.endTimestamp;
So my problem is as follow, I have a table in MySQL with a UserId column and an ObjectId column (its a many to many relationship), and what I would like is to have is a query that gives me the list of objects that user X and Y share. Not sure how to make the joins to make this happen.
Use query something like below using self join
Select columns from table t1 join table t2 on t1.objectid=t2.objectid where t1.userid=X and t2.userid=Y
I am relativly new to the SQL language. I can do a basic select, but for performance increase, I'd love to know if it is possible to merge the two queries I am doing at the moment into one.
Scenario: There are two tables. Table one has a few columns, one of them is a VARCHAR(45) named 'user', and another one is a INT which is called 'gid'. In the second table, there is a primary key column called 'gid' (INT) and a column called 'permissions' which is a TEXT column and it contains values seperated by ';'.
I have a user name, and want the text in the permissions column. The current way I do it is by fetching the gid of the first table, then doing a second query with the gid to get the permissions.
I've heard there are other ways to do this, and I have searched on Google, but I'm not sure what I should do.
EDIT:
Like this:
select t2.permissions
from table1 t1, table2 t2
where t1.user = '<SPECIFIED NAME>'
and t1.gid = t2.gid;
or you could use INNER JOIN syntax:
select t2.permissions
from table1 t1
inner join table2 t2 on t1.gid = t2.gid
where t1.user = '<SPECIFIED VALUE>'
To do this you use a JOIN. A join connects two tables in a select statement.
Like this
select *
from usertable u
join permissiontable p on u.gid = p.gid
This will give you all the columns from both tables with the id column joined. You can treat the joined table just like any table (eg select a sub-set of columns in the select list, add a where clause, etc).
You can read more about joins in any intro sql book or doing a google search.
the code below provide a result too much Infact i want to list the customer that never buy somethink How can i fix the code below
SELECT
webboard.listweb.id,
webboard.listweb.iditempro,
webboard.listweb.url,
webboard.listweb.useradddate,
webboard.listweb.expiredate,
webboard.prorecord.urlpostonweb
webboard.prorecord.urlpostonweb
FROM
webboard.listweb ,
webboard.prorecord
Where listweb.id Not In
(select webboard.prorecord.idlist From webboard.prorecord )
Using the syntax
FROM
webboard.listweb ,
webboard.prorecord
will perform a cartesian, or cross, join on the tables involved. So for every row in the table listweb all the rows in prorecord are displayed.
You need to use an INNER JOIN to only select the rows in listweb that have related rows in the prorecord table. What are the fields which identify the rows (your Primary Keys) and what is the name of the foreign key field in the prorecord table?
EDIT: Just re-read the question and comments and I see you want the rows in listweb which do not have an entry in prorecord
Your SELECT will then look like:
SELECT
webboard.listweb.id,
webboard.listweb.iditempro,
webboard.listweb.url,
webboard.listweb.useradddate,
webboard.listweb.expiredate,
webboard.prorecord.urlpostonweb
-- webboard.prorecord.urlpostonweb -- You have this field twice
FROM webboard.listweb LEFT JOIN webboard.prorecord
ON webboard.listweb.id = webboard.prorecord.idlist -- I'm guessing at the foreign key here
WHERE webboard.prorecord.idlist IS NULL