I have 3 tables inputs, posts, and posts_values:
I need to get all inputs with there values, and if value doesn't exist get null instead
like (input, value)
name: David, job:Developer, city:NULL
You need to LEFT JOIN the inputs table like this:
SELECT i.input_name, pv.value
FROM inputs as i
LEFT JOIN posts AS p ON i.form_id = p.id
LEFT JOIN post_values AS pv ON pv.b_id = p.id
AND pv.a_id = i.id;
This will give you:
| input_name | value |
|------------|-----------|
| name | David |
| job | Developer |
| city | (null) |
sql fiddle demo
Note that:, I assumed that a_id on the third table post_values is a foreign key for the column id in inputs table, so that each input name is matched with a value. Otherwise, you might need to make this a_id a surrogate key, and add an additional foreign key input_id.
You need to read about SQL joins and the difference between them, check this for example:
A Visual Explanation of SQL Joins
Related
I'm creating a website like SO. Now I want to know, when I write a comment under Jack's answer/question, what happens? SO sends a notification to Jack, right? So how SO finds Jack?
In other word, should I store author-user-id in the Votes/Comments tables? Here is my current Votes-table structure:
// Votes
+----+---------+------------+---------+-------+------------+
| id | post_id | table_code | user_id | value | timestamp |
+----+---------+------------+---------+-------+------------+
// ^ this column stores the user-id who has sent vote
// ^ because there is multiple Posts table (focus on the Edit)
Now I want to send a notification for post-owner. But I don't know how can I find him? Should I add a new column on Votes table named owner and store the author-id ?
Edit: I have to mention that I have four Posts tables (I know this structure is crazy, but in reality the structure of those Posts tables are really different and I can't to create just one table instead). Something like this:
// Posts1 (table_code: 1)
+----+-------+-----------+
| id | title | content |
+----+-------+-----------+
// Posts2 (table_code: 2)
+----+-------+-----------+-----------+
| id | title | content | author_id |
+----+-------+-----------+-----------+
// Posts3 (table_code: 3)
+----+-------+-----------+-----------+
| id | title | content | author_id |
+----+-------+-----------+-----------+
// Posts4 (table_code: 4)
+----+-------+-----------+
| id | title | content |
+----+-------+-----------+
But the way, Just some of those Post tables have author_id column (Because I have two Posts tables which are not made by the users). So, as you see, I can't create a foreign key on those Posts tables.
What I need: I want a TRIGGER AFTER INSERT on Votes table which send a notification to the author if there is a author_id column. (or a query which returns author_id if there is a author_id). Or anyway a good solution for my problem ...
Votes.post_id should be a foreign key into the Posts table. From there you can get Posts.author_id, and send the notification to that user.
With your multiple Posts# tables, you can't use a real foreign key. But you can write a UNION query that joins with the appropriate table depending on the table_code value.
SELECT p.author_id
FROM Votes AS v
JOIN Posts2 AS p ON p.id = v.post_id
WHERE v.table_code = 2
UNION
SELECT p.author_id
FROM Votes AS v
JOIN Posts3 AS p ON p.id = v.post_id
WHERE v.table_code = 3
Try to avoid storing data that you can get by following foreign keys, so that the information is only stored one place. If you run into performance problems because of excessive joining, you may need to violate this normalization principle, but only as a last resort.
I have two tables, "records", and "info".
The "records" table looks like:
mysql> SELECT * FROM records WHERE num = '7';
+-----+--------+----+------+-----+-----+------------+-----------+----------+---------------------+
| id | city | st | type | num | val | startdate | status | comments | updated |
+-----+--------+----+------+-----+-----+------------+-----------+----------+---------------------+
| 124 | Encino | CA | AAA | 7 | 1 | 1993-09-01 | allocated | | 2014-02-26 08:16:07 |
+-----+--------+----+------+-----+-----+------------+-----------+----------+---------------------+
and so on. Think of the "num" field in this table as a Company ID.
The "info" table contains information about certain companies, and uses that company id as a unique identifier. Not all companies listed in "records" will be in "info". An example of the "info" table:
mysql> SELECT * FROM info LIMIT 2;
+-----+-------+--------------------------+---------------------+
| org | name | description | updated |
+-----+-------+--------------------------+---------------------+
| 0 | ACME | | 2014-02-19 10:35:39 |
| 1 | AT&T | Some Phone Company, Inc. | 2014-02-18 15:29:50 |
+-----+-------+--------------------------+---------------------+
So "org" here will match "num" in the first table.
I want to be able to run a query that returns, on one line, everything but 'id', 'type' and 'val' from the 1st table, and IF APPLICABLE, the 'name' and 'description' from the 2nd table.
I can achieve what I want using this query:
SELECT city,st,num,startdate,status,comments,updated, \
( SELECT name FROM info WHERE org = '7') AS name, \
( SELECT description FROM info WHERE org = '7') AS description \
FROM records WHERE num = '7'
But I see at least two problems with it:
It seems inefficient to run two subqueries
When there is no record in "info", NULL is printed for the name and
description. I would like to print some string instead.
To address the first problem, I tried to return an array. But when no corresponding record exists in the "info" table, then I get nothing, not even the valid info from the "records" table. Here's my array query:
SELECT city,st,num,startdate,status,comments,updated,asinfo.name AS name,asinfo.description AS description \
FROM records, \
( SELECT name,description FROM info WHERE org = '7') AS asinfo \
WHERE num = '7'
This query works fine if a given company id exists in both tables.
To address the second problem, I tried various incantations of IFNULL and coalesce, to no avail.
I'd appreciate any insight.
Thanks.
Apply LEFT JOIN syntax:
SELECT
r.city,
r.st,
r.num,
r.startdate,
r.status,
r.comments,
r.updated,
IF(d.name IS NULL, 'Default', d.name) AS name,
IF(d.description IS NULL, 'Default', d.description) AS description
FROM
records AS r
LEFT JOIN info AS d ON r.num=d.org
WHERE
r.num='7'
that will work such way: LEFT JOIN looks into first table, and, if there are no corresponding records in second, it applies NULL. So you'll discover that with IF (or IFNULL) and do substitution of default string.
Use a LEFT JOIN to get null values when there's no matching row in the info table.
SELECT city,st,num,startdate,status,comments,updated,
IFNULL(name, 'Default Name') name,
IFNULL(description, 'Default Description') description
FROM records r
LEFT JOIN info i ON r.num = i.org
WHERE r.num = 7
It sounds like a simple LEFT JOIN from record to info will do the trick.
LEFT JOIN rather than JOIN in order to ensure you ALWAYS get all rows from the record table, and then the corresponding data in info table if a xref exists for that ID.
Whether using your sub-queries or using joins, if you always want to see all rows in record table, then you will always get NULLs corresponding to the info table where no xref exists. The only way to avoid that is to run some code that calls everything from record, and then iterates over the results to query info, to conditionally add to the record data.
Perhaps my failing is not knowing the terminology, but I am looking to do perform a JOIN on two tables with another table as the foreign keys.
So the id of an item on table A is used to get the foreign key from table B, which is used to get the textual representation from table C.
TABLE A
+----+------------+
| id | name |
+----+------------+
| 1 | comment |
+----+------------+
TABLE B
+-----------+------------------+
| object_id | cat_id_ref |
+-----------+------------------+
| 1 | 2 |
+-----------+------------------+
| 1 | 3 |
+-----------+------------------+
TABLE C
+---------+----------+
| cat_id | cat_type |
+---------+----------+
| 1 | Mean |
| 2 | Nice |
| 3 | Rude |
+---------+----------+
So the question is 2 part. what is this 1 step away foreign key lookup called, terminology, and for MySQL is this sub query? Best practice for this type of JOIN
The desired result would be:
+----+------------+------------+
| id | name | cat_type |
+----+------------+------------+
| 1 | veg 1 | Nice |
+----+------------+------------+
| 1 | veg 1 | Rude |
+----+------------+------------+
This looks like a many-to-many relationship. I refer to TABLE B as an "association" table (or, sometimes a "relationship" table, in some cases, this can be called a "membership" table.)
From a database design standpoint, we do "Entity Relationship Modeling". An "entity" is a person, place, thing, concept, or event that can be uniquely identified, is of interest to the business, and we can store information about. And we have "relationships" between those entities. When we have a relationship identified, we ask the right questions, to find out how many of one entity are related to another.
In your example, it looks like B implements an association, between entities A and C.
An "A" can be related to zero, one or more "C".
A "C" can be related to zero, one or more "A".
(This is where having a suitable name to identify an entity can make the model more understandable.)
To resolve the many-to-many, we implement a third table, which we could name "A_C" or "C_A", or it could have a totally different name. (For example, the association between "COMMITTEE" and "PERSON" might be called "MEMBERSHIP").
The new table, in your example, named "B"
A "B" is related to exactly one "A"
A "B" is related to exactly one "C"
An "A" is related to zero, one or more "B"
A "C" is related to zero, one or more "B"
The introduction of the "B" table resolves the many-to-many relationship into two "one-to-many" relationships.
(It looks like you already have an understanding of how a foreign key goes on the table on the "many" side of the relationship, and how that refers to the primary key (or unique key) of the entity table on "one" side of the relationship: the value of the foreign key is a copy of the primary key value from the other table.)
As to the second part of your question, here's an example of a query that will return the specified resultset:
SELECT a.id
, a.name
, c.veg_type
FROM A a
LEFT
JOIN B b
ON b.object_id = a.id
LEFT
JOIN C c
ON c.veg_id = b.veg_type_ref
(There are different queries that will return the same resultset; the big differences will be in the handling of rows that are "missing" (for example, a row in table A that has no "matching" row in table B. The other differences are performance, depending on cardinality, selectivity, available indexes, etc.)
These are all JOINs; there's no need to introduce a subquery to get the specified resultset.
It sounds like you're looking for something like this.
select A.*, C.veg_type
from A
inner join B
on A.id = B.object_id
inner join C
on C.veg_id = B.veg_id_ref
Basically, just a one - to - many relationship
Can a veg have more than 1 type? This example seems too simple to require the middle table.
select id,name, veg_type
from TableA
inner join TableB on Tablea.id = tableb.object_id
inner join TableC on tableb.veg_id_ref = tablec.id
I have a little SQL but I can't find the way to get back text just numbers. - revised!
SELECT if( `linktype` = "group",
(SELECT contactgroups.grname
FROM contactgroups, groupmembers
WHERE contactgroups.id = groupmembers.id ???
AND contactgroups.id = groupmembers.link_id),
(SELECT contactmain.contact_sur
FROM contactmain, groupmembers
WHERE contactmain.id = groupmembers.id ???
AND contactmain.id = groupmembers.link_id) ) AS adat
FROM groupmembers;
As now I have improved a bit gives back some info but ??? (thanks to minitech) indicate my problem. I can't see how could I fix... Any advice welcomed! Thansk
Contactmain (id, contact_sur, email2)
data:
1 | Peter | email#email.com
2 | Andrew| email2#email.com
Contactgroups (id, grname)
data:
1 | All
2 | Trustee
3 | Comitee
Groupmembers (id, group_id, linktype, link_id)
data:
1 | 1 | contact | 1
2 | 1 | contact | 2
3 | 2 | contact | 1
4 | 3 | group | 2
And I would like to list out who is in the 'Comitee' the result should be Andrew and Trustee if I am right:)
It does look a bit redundant on the join since you are implying both the ID and Link_ID columns are the same value. Since BOTH select values are derived from a qualification to the group members table, I have restructured the query to use THAT as the primary table and do a LEFT JOIN to each of the other tables, anticipating from your query that the link should be found from ONE or the OTHER tables. So, with each respective LEFT JOIN, you will go through the GroupMembers table only ONCE. Now, your IF(). Since the group members is the basis, and we have BOTH tables available and linked, we just grab the column from one table vs the other respectively. I've included the "linktype" too just for reference purposes. By using the STRAIGHT_JOIN will help the engine from trying to change the interpretation of how to join the tables.
SELECT STRAIGHT_JOIN
gm.linktype,
if( gm.linktype = "group", cg.grname, cm.contact_sur ) ADat
from
groupmembers gm
left join contactgroups cg
ON gm.link_id = cg.id
left join contactmain cm
ON gm.link_id = cm.id
If contactgroups.id must equal groupmembers.id but must also equal 2, that's redundant and also probably where your problem is. It works fine as you've written it: http://ideone.com/7EGLZ so without knowing what it's actually supposed to do I can't help more.
EDIT: I'm unfamiliar with the comma-separated FROM, but it gives the same result since you don't select anything from the other table so it doesn't really matter.
I've got a core table and and 3 tables that extend the 'core' table in different ways.
I'm working with MLS data and I have a 'common' table that contains information common to all mls listings and then a table that has specifically "residential" information, one for "commercial",etc... I have been using mls number to join a single table when I know a listing when the property type is known, but for searching I want to join all of them and have the special fields available for search criteria (not simply searching the common table).
What type of join will give me a dataset that will contain all listings (including the extended fields in the idx tables) ?
For each Common table record there is a single corresponding record in ONLY ONE of the idx tables.
___________
| |
| COMMON |
| |
|___________|
_|_
|
___________________|_____________________
_|_ _|_ _|_
_____|_____ _____|______ ____|______
| | | | | |
| IDX1 | | IDX2 | | IDX3 |
| | | | | |
|___________| |____________| |___________|
If you want everything in one row, you can use something like this format. Basically it gives you all the "Common" fields, then the other fields if there is a match otherwise NULL:
SELECT Common.*,
Idx1.*,
Idx2.*,
Idx3.*
FROM Common
LEFT JOIN Idx1
ON Idx1.MLSKey = Common.MLSKey
LEFT JOIN Idx2
ON Idx2.MLSKey = Common.MLSKey
LEFT JOIN Idx3
ON Idx3.MLSKey = Common.MLSKey
Bear in mind it's better to list out fields than to use the SELECT * whenever possible...
Also I'm assuming MySQL syntax is the same as SQL Server, which is what I use.
I have a similar set up of tables where the table 'jobs' is the core table.
I have this query that selects certain elements from each of the other 2 tables:
SELECT jobs.frequency, twitterdetails.accountname, feeds.feed
FROM jobs
JOIN twitterdetails ON twitterdetails.ID = jobs.accountID
JOIN feeds ON jobs.FeedID = feeds.FeedID
WHERE jobs.username ='".$currentuser."';");
So, as you can see, no specific JOIN, but the linking fields defined. You'd probably just need an extra JOIN line for your set up.
Ugly solution / poor attempt / may have misunderstood question:
SELECT common.*,IDX1.field,NULL,NULL FROM COMMON
LEFT JOIN IDX1 ON COMMON.ID = IDX1.ID
WHERE TYPE="RESIDENTIAL"
UNION ALL
SELECT common.*,NULL,IDX2.field,NULL FROM COMMON
LEFT JOIN IDX2 ON COMMON.ID = IDX2.ID
WHERE TYPE="RESIDENTIAL"
UNION ALL
SELECT common.*,NULL,NULL,IDX3.field FROM COMMON
LEFT JOIN IDX3 ON COMMON.ID = IDX3.ID
WHERE TYPE="INDUSTRIAL"
Orbit is close. Use inner join, not left join. You don't want common to show up in the join if it does not have a row in idx.
You MUST union 3 queries to get the proper results assuming each record in common can only have 1 idx table. Plug in "NULL" to fill in the columns that each idx table is missing so they can be unioned.
BTW your table design is good.