nested select to fetch date in SQL - mysql

Question

There is 3 table.
items_table:
iid
ititle
itext
tag_list_table
tid
tname;
tag_ref_table
iid
tid //foreign key for tag_list_table
per record in tag_list_table have one or more tags.
for example:
tag_list_table:
iid=1,itltle="this is title";itext="this is full text";
iid=2,itltle="this is title2";itext="this is full text2";
tag_list_table
tid=1 tname="red"
tid=2 tname="green"
tid=3 tname="yellow"
tid=4 tname="orange"
tag_ref_table
iid=1 tid=1
iid=1 tid=2
iid=1 tid=3
iid=2 tid=4
I want to have result like this:
row1:
1-
this is title
this is full text
red,green,yellow
row2:
2-
this is title2
this is full text2
orange
I tried these:
SELECT i.ititle,i.itext,t.tname
FROM tag_ref_table as i
LEFT JOIN tag_ref_table t WHERE tid=iid
LEFT JOIN tag_list_table r WHERE tid=??????....

Answer 1

If the "red,green,yellow" values are concatenated into a single string, in MySQL you could use GROUP_CONCAT function like this:
SELECT i.*, group_concat(l.tname SEPARATOR ',') as names
FROM items_table as i
JOIN tag_ref_table r ON r.iid=i.iid
JOIN tag_list_table l ON r.tid= l.tid
GROUP BY i.iid
Documentation: http://dev.mysql.com/doc/refman/5.7/en/group-by-functions.html#function_group-concat
Further examples: http://www.giombetti.com/2013/06/06/mysql-group_concat/

Answer 2

You are looking for GROUP_CONCAT
SELECT it.ititle, it.itext, GROUP_CONCAT(tlt.tname)
FROM tag_ref_table as trt
INNER JOIN items_table it ON trt.iid = it.iid
INNER JOIN tag_list_table tlt ON trt.tid = tlt.tid
GROUP BY it.ititle, it.itext;
SqlFiddle here
Unless your tag_ref_table Junction table allows for NULLS (which would be unusual in such a table), I would recommend switching to an INNER JOIN.

Answer 3

You can use CONCAT() and GROUP_CONCAT() in your select:
SELECT CONCAT(t1.iid,'- ',t1.ititle,' ',t1.itext,' ',t1.tname) FROM
(SELECT i.iid,i.ititle,i.itext,GROUP_CONCAT(r.tname) tname
FROM items_table i
INNER JOIN tag_ref_table t ON i.iid = t.iid
INNER JOIN tag_list_table r ON t.tid = r.tid
GROUP BY i.ititle,i.itext,i.iid)t1;
you can see complete answer in:
SQL Fiddle

Answer 4

DROP TABLE items;
CREATE TABLE items
(item_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY
,item_title VARCHAR(20)
,item_text VARCHAR(20) NOT NULL
);
DROP TABLE tags;
CREATE TABLE tags
(tag_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY
,tag_name VARCHAR(12) NOT NULL
);
DROP TABLE items_tags;
CREATE TABLE items_tags
(item_id INT NOT NULL
,tag_id INT NOT NULL
,PRIMARY KEY(item_id,tag_id)
);
INSERT INTO items VALUES
(1,"this is title1","this is full text1"),
(2,"this is title2","this is full text2");
INSERT INTO tags VALUES
(1,"red"),
(2,"green"),
(3,"yellow"),
(4,"orange");
INSERT INTO items_tags VALUES
(1,1),
(1,2),
(1,3),
(2,4);
mysql> SELECT * FROM items;
+---------+----------------+--------------------+
| item_id | item_title | item_text |
+---------+----------------+--------------------+
| 1 | this is title1 | this is full text1 |
| 2 | this is title2 | this is full text2 |
+---------+----------------+--------------------+
2 rows in set (0.00 sec)
SELECT * FROM tags;
+--------+----------+
| tag_id | tag_name |
+--------+----------+
| 1 | red |
| 2 | green |
| 3 | yellow |
| 4 | orange |
+--------+----------+
SELECT * FROM items_tags;
+---------+--------+
| item_id | tag_id |
+---------+--------+
| 1 | 1 |
| 1 | 2 |
| 1 | 3 |
| 2 | 4 |
+---------+--------+
SELECT i.*
, GROUP_CONCAT(t.tag_name ORDER BY t.tag_id) tags
FROM items i
JOIN items_tags it
ON it.item_id = i.item_id
JOIN tags t
ON t.tag_id = it.tag_id
GROUP
BY i.item_id;
+---------+----------------+--------------------+------------------+
| item_id | item_title | item_text | tags |
+---------+----------------+--------------------+------------------+
| 1 | this is title1 | this is full text1 | red,green,yellow |
| 2 | this is title2 | this is full text2 | orange |
+---------+----------------+--------------------+------------------+
Note that the GROUP/GROUP_CONCAT part can be performed just as well (if not better) at the application level (e.g. a simple PHP loop acting upon an ordered array)