How to select adjacent columns in MySQL
My table: vocabulary
id word
--------------------------
1 hello
2 hello
3 how
4 how
5 how
6 are
7 hello
8 hello
9 are
10 are
11 are
12 are
13 hello
I want to select id from vocabulary where id=$id and {all rows that are both the same word and adjacent}
Note: I want both of them: [id = $ id] and [all lines that are the same word and next]
Actually I need a query SELECT
to do something like this: (three examples)
-
$id=1
, result: 1,2 // [1 for$id=1
] - [2 for word 1 and 2 are the same and adjacent] -
$id=6
, result: 6 // [6 for$id=6
] -
$id=10
, result: 9,10,11,12 // [10 for$id=10
] - [9,11,12 for word 10 is the same with 9, 11,12]
This is a solution using variables:
SELECT id, word
FROM (
SELECT id,
@rnk:= CASE WHEN @word = word THEN @rnk
ELSE @rnk + 1
END AS rnk,
@word:= word AS word
FROM vocabulary, (SELECT @rnk:=0) as vars
ORDER BY id ) s
WHERE s.rnk = (
SELECT rnk
FROM (
SELECT id,
@r:= CASE WHEN @w = word THEN @r
ELSE @r + 1
END AS rnk,
@w:= word AS word
FROM vocabulary, (SELECT @r:=0) as vars
ORDER BY id ) t
WHERE id = 10) -- 10 is equal to $id
The same query is repeated two times due to missing CTE
MySQL. @rnk
and @r
are used to identify islands with continuous values word
in a table vocabulary
.
The second query selects an island value (for example, @r = 5
for id = 10
), and the first uses that value to select all records belonging to the same island.
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Here's a basic template that you can adapt to suit your purpose ...
DROP TABLE IF EXISTS my_table;
CREATE TABLE my_table
(id INT NOT NULL AUTO_INCREMENT PRIMARY KEY
,word VARCHAR(12) NOT NULL
);
INSERT INTO my_table VALUES
(1 ,'hello'),
(2 ,'hello'),
(3 ,'how'),
(4 ,'how'),
(5 ,'how'),
(6 ,'are'),
(7 ,'hello'),
(8 ,'hello'),
(9 ,'are'),
(10,'are'),
(11,'are'),
(12,'are'),
(13,'hello');
SELECT a.id start
, MIN(c.id) end
FROM my_table a
LEFT
JOIN my_table b
ON b.id = a.id - 1
AND b.word = a.word
LEFT
JOIN my_table c
ON c.id >= a.id
AND c.word = a.word
LEFT
JOIN my_table d
ON d.id = c.id + 1
AND d.word = a.word
WHERE b.id IS NULL
AND c.id IS NOT NULL
AND d.id IS NULL
GROUP
BY a.id;
+-------+------+
| start | end |
+-------+------+
| 1 | 2 |
| 3 | 5 |
| 6 | 6 |
| 7 | 8 |
| 9 | 12 |
| 13 | 13 |
+-------+------+
As McAdam331 suggests, one way to extend this idea is as follows:
SELECT *
FROM vocabulary
JOIN tmpTable
WHERE id BETWEEN tmpTable.start AND tmpTable.end
AND tmpTable.start = $id;
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