I have four tables. The first describing a mix of items. The second is a linking table between the mix, and the items. The third is the item table, and the fourth holds lot information - lot number, and when that lot starts being used.
mix
mixID | mixName
----------------
1 | Foxtrot
2 | Romeo
mixLink
mixID | itemID
----------------
1 | 1
1 | 2
1 | 3
item
itemID| itemName
----------------
1 | square
2 | triangle
3 | hexagon
itemLots
itemID| lotNo | startDate
-------------------------
1 | 22/5/3| 22/07/16
2 | 03/5 | 25/07/16
2 | 04/19 | 12/08/16
3 | 15/0 | 05/08/16
Now, I need to be able to fetch the information from the database, which details all the items from a mix, as well as the most recently used lot number, something like this:
itemName | lotNo
----------------
square | 22/5/3
triangle | 04/19
hexagon | 15/0
I've tried a dozen different mixes of joins, group by's, maxes, subqueries, and havings; all to no avail. Any help would be much appreciated, I've been pulling my hair out for hours, and I feel like my fingernails are just scraping at the solution!
This will give you the result you're after and will perform pretty well if you have your indexes done properly. I'm not sure how you're meaning to reference mix as it's not apparent in your sample output but I've included it in the WHERE clause so hopefully you can understand where you would use it.
SELECT i.itemName
, (SELECT il.lotNo FROM itemLots il
WHERE il.itemID=i.itemID
ORDER BY il.startDate desc
LIMIT 1) as lotNo
FROM item i
JOIN mixLink ml ON ml.itemID=i.itemID
JOIN mix m ON m.mixID=ml.mixID
WHERE m.mixName="Foxtrot";
Related
I have a page that displays a list of projects. With each project is displayed the following data retrieved from a mysqli database:
Title
Subtitle
Description
Part number (1 of x)
The total number of photos associated with that project
A randomly selected photo from the project
A list of tags
Projects are displayed 6 per page using a pagination system
As this is based on an old project of mine, it was originally done with sloppy code (I was just learning and did not know any better) using many queries. Three, in fact, just for items 5-7, and those were contained within a while loop that worked with the pagination system. I'm now quite aware that this is not even close to being the right way to do business.
I am familiar with INNER JOIN and the use of subqueries, but I'm concerned that I may not be able to get all of this data using just one select query for the following reasons:
Items 1-4 are easy enough with a basic SELECT query, BUT...
Item 5 needs a SELECT COUNT AND...
Item 6 needs a basic SELECT query with an ORDER by RAND LIMIT 1 to
select one random photo out of all those associated with each project
(using FilesystemIterator is out of the question, because the photos
table has a column indicating 0 if a photo is inactive and 1 if it is
active)
Item 7 is selected from a cross reference table for the tags and
projects and a table containing the tag ID and names
Given that, I'm not certain if all this can (r even should for that matter) be done with just one query or if it will need more than one query. I have read repeatedly how it is worth a swat on the nose with a newspaper to nest one or more queries inside a while loop. I've even read that multiple queries is, in general, a bad idea.
So I'm stuck. I realize this is likely to sound too general, but I don't have any code that works, just the old code that uses 4 queries to do the job, 3 of which are nested in a while loop.
Database structure below.
Projects table:
+-------------+---------+----------+---------------+------+
| project_id | title | subtitle | description | part |
|---------------------------------------------------------|
| 1 | Chevy | Engine | Modify | 1 |
| 2 | Ford | Trans | Rebuild | 1 |
| 3 | Mopar | Diff | Swap | 1 |
+-------------+---------+----------+---------------+------+
Photos table:
+----------+------------+--------+
| photo_id | project_id | active |
|--------------------------------|
| 1 | 1 | 1 |
| 2 | 1 | 1 |
| 3 | 1 | 1 |
| 4 | 2 | 1 |
| 5 | 2 | 1 |
| 6 | 2 | 1 |
| 7 | 3 | 1 |
| 8 | 3 | 1 |
| 9 | 3 | 1 |
+----------+------------+--------+
Tags table:
+--------+------------------+
| tag_id | tag |
|---------------------------|
| 1 | classic |
| 2 | new car |
| 3 | truck |
| 4 | performance |
| 5 | easy |
| 6 | difficult |
| 7 | hard |
| 8 | oem |
| 9 | aftermarket |
+--------+------------------+
Tag/Project cross-reference table:
+------------+-----------+
| project_id | tag_id |
|------------------------|
| 1 | 1 |
| 1 | 3 |
| 1 | 4 |
| 2 | 2 |
| 2 | 5 |
| 3 | 6 |
| 3 | 9 |
+------------+-----------+
I'm not asking for the code to be written for me, but if what I'm asking makes sense, I'd sincerely appreciate a shove in the right direction. Often times I struggle with both the PHP and MySQLi manuals online, so if there's any way to break this down, then fantastic.
Thank you all so much.
You're able to do subqueries inside your SELECT clause, like this:
SELECT
p.title, p.subtitle, p.description, p.part,
(SELECT COUNT(photo_id) FROM Photos where project_id = p.project_id) as total_photos,
(SELECT photo_id FROM Photos where project_id = p.project_id ORDER BY RAND LIMIT 1) as random_photo
FROM projects as p
Now, for the list of tags, as it returns more than one row, you can't do a subquery and you should do one query for every project. Well, in fact you can if you return all the tags in some kind of concatenation, like a comma separated list: tag1,tag2,tag3... but I don't recommend this one time that you will need to explode the column value. Do it only if you have many many projects and the performance to retrieve the list of tags for each individual project is fairly low. If you really want, you can:
SELECT
p.title, p.subtitle, p.description, p.part,
(SELECT COUNT(photo_id) FROM Photos where project_id = p.project_id) as total_photos,
(SELECT photo_id FROM Photos where project_id = p.project_id ORDER BY RAND LIMIT 1) as random_photo,
(SELECT GROUP_CONCAT(tag SEPARATOR ', ') FROM tags WHERE tag_id in (SELECT tag_id FROM tagproject WHERE project_id = p.project_id)) as tags
FROM projects as p
As you said from item 1 to 4 you already have the solution.
Add to the same query a SQL_CALC_FOUND_ROWS instead of a SELECT COUNT to solve the item 5.
For the item 6 you can use a subquery or maybe a LEFT JOIN limiting to one result.
For the latest item you can also use a subquery joining all the tags in a single result (separated by comma for instance).
I'm working on simple application using PHP and MySQL. Up to this point we needed to display items from database in HTML table. Simple pagination was implemented as well. It looks something like this:
+----+---------------------+
| 1 | Item 1 |
+----+---------------------+
| 2 | Item 2 |
+----+---------------------+
| 3 | Item 3 |
+----+---------------------+
| 4 | Item 4 |
+----+---------------------+
....
+----+---------------------+
| 5 | Item 25 |
+----+---------------------+
Not a rocket science. Now we add new functionality so we can (optionally) group items - We really create a 'lot' of identical items. We decided to add new column in database called groupID - which can be number or NULL for items not contained in any group. On web page we must display it as one element which expands when you click on it.
+----+---------------------+
| 1 | Item 1 |
+----+---------------------+
| 2 | Item 2 |
+----+---------------------+
| 3 | Item 3 |
+----+---------------------+
| 4 | Group 1 (Expanded) |
+----+---------------------+
| Group 1 Item 1 |
+---------------------+
| Group 1 Item 2 |
+---------------------+
| Group 1 Item 3 |
+----+---------------------+
....
+----+---------------------+
| 25| Item 25 |
+----+---------------------+
As you can see Number of items on one page may vary so we must treat items in group as one item, so simple 'limit 25' not working anymore. I wonder if I can make some clever mysql query which will work this way. I rather want to avoid to create new table in database which consists groups and relation to item table, because most of the groups will have only 1 Item. I don't believe this functionality will be used a lot, but You know - client. Also this system works on production for some time so I'd rather avoid such changes. Any Idea how to make it work? Also please keep it simple as possible, because this example is simplified. Real query is already bit complicated.
I also want avoid parsing it via PHP code, because it's just dumb to query all few thousands of rows and then discard all but 25-50 elements.
As you said some group ids can be null, I thought we should fix that.
When null, we use the item id to for the group and we use a prefix to make sure our new group_id is unique.
This is my solution using subqueries (not pretty, but seems to work):
SELECT
i1.id,
i1.itemgroup1
FROM (
SELECT
items.id,
IF(ISNULL(items.group_id),
CONCAT('alone-', items.id),
CONCAT('group-', items.group_id)) as itemgroup1
FROM
items
) as i1
RIGHT JOIN (
SELECT
items.id,
IF(ISNULL(items.group_id),
CONCAT('alone-', items.id),
CONCAT('group-', items.group_id)) as itemgroup2
FROM
items
GROUP BY itemgroup2
LIMIT 2
) as i2 on i2.itemgroup2 = i1.itemgroup1
** UPDATE **
Removed
WHERE
items.group_id IS NOT NULL
I am trying to come up with a single result set from two tables in a way that I have never done, and I am having a little bit of trouble figuring out how to do it or even what to search for in these forums. Consider the following hypothetical table data:
Table1
----------------------------------
ID | Name
----------------------------------
1 | aa
2 | bb
3 | cc
4 | dd
5 | ee
Table2
----------------------------------
ID | Table1_ID | Value
----------------------------------
1 | 1 | good
2 | 2 | Dumb
3 | 3 | Fat
4 | 4 | Wet
5 | 5 | High
6 | 1 | Thin
7 | 2 | Tall
8 | 3 | Goofy
9 | 4 | Rich
10 | 5 | Funny
I am looking for a query or method that allows me to end up with the following result set:
Code:
aa | bb | cc | dd | ee
---------------------------------------------------------------
good | Dumb | Fat | Wet | High
Thin | Tall | Goofy | Rich | Funny
Essentially, I want the ability to take the list of names from Table1, transpose them into column headers, then put all of Table2's values into their respective columns with the ability to sort on any column. Is this possible?
Of course this can be done in SQL. But it is tricky. As the data is written in the question, you can group by t2.id in groups of 5. After that, the query is just conditional aggregation.
select max(case when t2.table1_Id = 1 then value end) as aa,
max(case when t2.table1_Id = 2 then value end) as bb,
max(case when t2.table1_Id = 3 then value end) as cc,
max(case when t2.table1_Id = 4 then value end) as dd,
max(case when t2.table1_Id = 5 then value end) as ee
from table2 t2
group by cast(t2.id - 1 / 5 as int);
Having values be implicitly related by their ids seems like a really, really bad database design. There should be some sort of entity id that combines them.
You've got two problems here:
1) Using values as column names can't be done in a clean way
2) You want to split table2.value in 2 rows: Which of the values should be on which row? Gordon Linoff uses the table2.id field for this, but if it's auto increment and your data gets some adds/deletes later on that rhythm will get broken.
There's been similar questions before. This one has an answer that gets pretty close:
mysql select dynamic row values as column names, another column as value
Here they generate the string for the query and make a prepared statement out of it.
I have two tables in my database
table: products table: companies
+-----------+------------+ +------+------------+
| name | company_id | | id | name |
+-----------+------------+ +------+------------+
|Product 1 | 1 | | 1 | Company 1 |
|Product 2 | 1 | | 2 | Company 2 |
|Product 3 | 3 | | 3 | Company 3 |
|Product 4 | 1 | | 4 | Company 4 |
|Product 5 | 1 | | ... |
|Product 6 | 3 | +------+------------+
|Product 7 | 2 |
|... |
+-----------+------------+
Now I have to make company-selector (filtering products by company) using HTML SELECT element with names of all companies from table companies and COUNT of products after company name in the list.
So, my goal is to get SELECT options like this:
Company 1 (4)
Company 2 (1)
Company 3 (2)
Company 4 (0)
(note: counts inside the brackets are from example above)
What have I tried so far?
I was using mysql_* functions earlier and later it was mysqli procedural model. I can do this manually with one query for companies and another one inside while block to get COUNT of elements (filtered by current company's id in the loop). Now I'm trying to work with PDO object which is something new for me and I'm not very familiar with it.
Question
Is it somehow possible to get COUNT with one query (using JOINs or something)? If not, how I can do it with query inside the loop (old way) using my $dbPDO object? Or any other way?
I've looked some examples here but nothing could adapt to fit my requirements. Maybe I missed something but working with PDO is still painful for me (and I must learn it ASAP).
Looking at my own question, I don't think it's something hard, but the worst thing, I can't find solution by myself.
At the end, I have solution in mysqli, just I don't like it so much and think there's easier way of making this task done!
I thought this question is something I need but still don't understand that query in answer.
So far I have this code and have no idea how to make it counts products:
$dbPDO = new PDO('mysql:dbname=comixdb;host=localhost', 'root', '');
$sel_cat = ''; # selector HTML
$sql = "SELECT * FROM categories ORDER BY name";
foreach ($dbPDO->query($sql) as $row) {
$sel_cat .= "<option value=\"{$row['id']}\">{$row['name']}</option>";
}
$sel_cat = "<select><option value=\"*\">All categories</option>$sel_cat</select>";
echo $sel_cat;
Hope I've clarified question enough. Any help would be appreciated.
What you need can be done in SQL:
SELECT companies.name, SUM(IF(company_id IS NULL, 0, 1)) AS products
FROM companies
LEFT JOIN products ON (companies.id = products.company_id)
GROUP BY companies.id;
The LEFT JOIN ensures that all companies get selected, and the SUM ensures that the count is correct (a simple COUNT would return 1 for companies with no products).
In a MySQL query, when using the DISTINCT option, does ORDER BY apply after the duplicates are removed? If not, is there any way to make it do so? I think it's causing some issues with my code.
EDIT:
Here's some more information about what's causing my problem. I understand that, at first glance, this order would not be important, since I am dealing with duplicate rows. However, this is not entirely the case, since I am using an INNER JOIN to sort the rows.
Say I have a table of forum threads, containing this data:
+----+--------+-------------+
| id | userid | title |
+----+--------+-------------+
| 1 | 1 | Information |
| 2 | 1 | FAQ |
| 3 | 2 | Support |
+----+--------+-------------+
I also have a set of posts in another table like this:
+----+----------+--------+---------+
| id | threadid | userid | content |
+----+----------+--------+---------+
| 1 | 1 | 1 | Lorem |
| 2 | 1 | 2 | Ipsum |
| 3 | 2 | 2 | Test |
| 4 | 3 | 1 | Foo |
| 5 | 2 | 3 | Bar |
| 6 | 3 | 5 | Bob |
| 7 | 1 | 2 | Joe |
+----+----------+--------+---------+
I am using the following MySQL query to get all threads, then sort them based on the latest post (assuming that posts with higher ids are more recent:
SELECT t.*
FROM Threads t
INNER JOIN Posts p ON t.id = p.threadid
ORDER BY p.id DESC
This works, and generates something like this:
+----+--------+-------------+
| id | userid | title |
+----+--------+-------------+
| 1 | 1 | Information |
| 3 | 2 | Support |
| 2 | 1 | FAQ |
| 3 | 2 | Support |
| 2 | 1 | FAQ |
| 1 | 1 | Information |
| 1 | 1 | Information |
+----+--------+-------------+
However, as you can see, the information is correct, but there are duplicate rows. I'd like to remove such duplicates, so I used SELECT DISTINCT instead. However, this yielded the following:
+----+--------+-------------+
| id | userid | title |
+----+--------+-------------+
| 3 | 2 | Support |
| 2 | 1 | FAQ |
| 1 | 1 | Information |
+----+--------+-------------+
This is obviously wrong, since the "Information" thread should be on top. It would seem that using DISTINCT causes the duplicates to be removed from the top to the bottom, so only the final rows are left. This causes some issues in the sorting.
Is this the case, or am I analyzing things incorrectly?
Two things to understand:
Generally speaking, resultsets are unordered unless you specify an ORDER BY clause; to the extent that you specify a non-strict order (i.e. ORDER BY over non-unique columns), the order in which records that are equal under that ordering appear within the resultset is undefined.
I suspect you may be specifying such a non-strict order, which is the root of your problems: ensure that your ordering is strict by specifying ORDER BY over a set of columns that is sufficient to uniquely identify each record for which you care about its final position in the resultset.
DISTINCT may use GROUP BY, which causes the results to be ordered by the grouped columns; that is, SELECT DISTINCT a, b, c FROM t will produce a resultset that appears as though ORDER BY a, b, c has been applied. Again, specifying a sufficiently strict order to meet your needs will override this effect.
Following your update, bearing in mind my point #2 above, it is clear that the effect of grouping the results to achieve DISTINCT makes it impossible to then order by the non-grouped column p.id; instead, you want:
SELECT t.*
FROM Threads t INNER JOIN Posts p ON t.id = p.threadid
GROUP BY t.id
ORDER BY MAX(p.id) DESC
DISTINCT informs MySQL how to build a rowset for you, ORDER BY gives a hint how this rowset should by presented. So the answer is: DISTINCT first, ORDER BY last.
The order in which DISTINCT and ORDER BY are applied, in most cases, will not affect the final output.
However, if you also use GROUP BY, this will affect the final output. In this case, the ORDER BY is performed after the GROUP BY, which will return unexpected results (assuming you expect the sort to be performed before the grouping).