I'm trying to create a search function in different tables using UNION and what happened is that the id's are duplicating making the search go wrong. How can I merge different tables into one while no id's are in common?
Here is the example
table1
id name desc
1 henry post
2 albert doth
3 jun cloth
table2
id name desc
1 kin revenge
2 pot eve
The result SHOULD be like this
id name desc
1 henry post
2 albert doth
3 jun cloth
4 kin revenge
5 pot eve
Please help me. Thanks.
In most databases, you would add a new id using the ANSI standard row_number() function:
select row_number() over (order by which, id) as newid, name, description
from (select 1 as which, t1.* from table1 t1 union all
select 2 as which, t2.* from table2 t2
) t;
Note that desc is a really bad name for a column, because it is a SQL keyword and usually a reserved word.
EDIT:
MySQL doesn't support this ANSI standard functionality. Instead, use variables:
select (#rn := #rn + 1) as newid, name, description
from (select 1 as which, t1.* from table1 t1 union all
select 2 as which, t2.* from table2 t2
) t cross join
(select #rn := 0) vars
order by which, id;
I've include the order by so the rows remain in the same order that you seem to want them in -- rows from the first table followed by rows from the second table. If you don't care about the order, just drop the order by.
For SQLite, the calculation is much more painful:
with cte as (
select 1 as which, t1.* from table1 t1 union all
select 2 as which, t2.* from table2 t2
)
select (select count(*)
from cte cte2
where cte2.which < cte.which or (ct2.which = cte.which and cte2.id <= cte.id
) as id,
name, description
from cte;
In MySql, you can simulate the row_number() function of Sql Server and Oracle using a mutating variable hack:
set #rownum := 0;
SELECT #rownum:=#rownum+1 AS` row_number`, `name`, `desc`
FROM
(
SELECT `name`, `desc` FROM table1
UNION
SELECT `name`, `desc` FROM table2
) AS x;
SqlFiddle
It looks like you have to Generate Id's so you can make you Union query as Sub select and generate Id's in Outer Query
MySQL does not have any system function like SQL Server’s row_number () to generate the row number for each row. However, it can be generated using the variable in the SELECT statement
SET #row_number:=0;
SELECT #row_number:=#row_number+1 As Id,
NAME,
desc
FROM (SELECT NAME,desc
FROM table1
UNION ALL
SELECT NAME,desc
FROM table2
UNION ALL
........
........) A
Order by NAME -- Change the column in Order by in which order you want to create New ID's
Related
I have a table as follows and what I want is to use get the initial row with least id of each uid group.
The table is as follows
_id uid type
1 a a
2 b bbb #satisfied
3 b ccc
4 b aaa #satisfied
5 a aaa #satisfied
6 b eee
I can already get the initial row using the following correlated subquery
SELECT *
FROM table
WHERE _id IN (
SELECT MIN(_id)
FROM table
WHERE type IN ('aaa','bbb')
GROUP BY uid
);
However, I want the 4th column shown the count of rows satisfied the condition (type IN ('aaa','bbb')), as cnt shown below:
_id uid type cnt
5 a aaa 1
2 b bbb 2
I think I can count this use several joins and then join the result to my code...But this is ugly...Is there any elegant way to achieve this...
You can try this:
SELECT t1.*, t2.cnt
FROM table t1 INNER JOIN (
SELECT MIN(_id) AS id, COUNT(_id) AS cnt
FROM table
WHERE type IN ('aaa','bbb')
GROUP BY uid
) t2 ON t1._id = t2.id
ORDER BY t1.uid
If you are running MySQL 8.0, you can just use window functions for this:
select _id, uid, type, cnt
from (
select
t.*,
count(*) over(partition by uid) cnt,
row_number() over(partition by uid order by _id) rn
from mytable t
where type in ('aaa', 'bbb')
) t
where rn = 1
You can do this without a subquery. In MySQL 8+, you can use this logic:
SELECT DISTINCT MIN(_id) OVER (PARTITION BY uid) as _id,
uid,
FIRST_VALUE(type) OVER (PARTITION BY uid ORDER BY _id) as type,
COUNT(*) OVER (PARTITION BY uid) as cnt
FROM table
WHERE type IN ('aaa', 'bbb');
Unfortunately, MySQL doesn't have a "first" aggregation function, but there is a trick if you like:
SELECT MIN(_id) as _id, uid,
SUBSTRING_INDEX(GROUP_CONCAT(type ORDER BY _id), ',', 1) as type,
COUNT(*) as cnt
FROM table
WHERE type IN ('aaa', 'bbb')
GROUP BY uid;
Lets say I have two subqueries in a UNION statement like so:
(
SELECT *
FROM users
ORDER BY registration_date
)
UNION ALL
(
SELECT *
FROM food
ORDER BY popularity
)
The output is the following:
Bob
Alice
Steve
Mark
...
Sandwich
Pizza
Burger
Fries
...
Is it possible to output them in an alternating fashion, such that the output is:
Bob
Sandwich
Alice
Pizza
Steve
Burger
Mark
Fries
...
Each query output is thousands of items.
You could use row_number() if you are running MySQL 8.0:
(select name, 1 src, row_number() over(order by registration_date) rn from users)
union all
(select name, 2, row_number() over(order by popularity) from food)
order by rn, src
In each unioned subquery, we use row_number() to rank the records, and add another column, called src to identify from which table the record comes from.
Then all that is left to do is order by the assigned row_number(), using the additional column to alternate the records.
Note that I modified your query to enumerate the columns being selected in the subqueries; select * is generally not a good practice, especially with union all, which requires both datasets to have the same number of columns (with equivalent datatypes).
Just in case you are not using MySQL 8+, you can still simulate ROW_NUMBER using a correlated count query:
(
SELECT 1 AS idx, name,
(SELECT COUNT(*) FROM users u2 WHERE u2.registration_date < u1.registration_date) rn
FROM users u1
)
UNION ALL
(
SELECT 2, name,
(SELECT COUNT(*) FROM food f2 WHERE f2.popularity < f1.popularity) rn
FROM food f1
)
ORDER BY
rn,
idx;
DECLARE #id INT;
SET #id :=0;
SELECT id,t.* FROM(
SELECT u.*,(#id:=#id+1) AS id
FROM users u
ORDER BY registration_date
)
UNION ALL
(
SELECT f.*,(#id:=#id+2) AS id
FROM food f
ORDER BY popularity
)t
ORDER BY id;
having a list of people like:
name date_of_birth
john 1987-09-08
maria 1987-09-08
samuel 1987-09-09
claire 1987-09-10
jane 1987-09-10
rose 1987-09-12
...
How can I get a result view using SQL of how many people are born up to that date, like the output for that table should be:
date count
1987-09-08 2
1987-09-09 3
1987-09-10 5
1987-09-11 5
1987-09-12 6
...
Thanks!
Here is another way, in addition to Gordon's answer. It uses joins:
SELECT
t1.date_of_birth,
COUNT(*) AS count
FROM (SELECT DISTINCT date_of_birth FROM yourTable) t1
INNER JOIN yourTable t2
ON t1.date_of_birth >= t2.date_of_birth
GROUP BY
t1.date_of_birth;
Note: I left out a step. Apparently you also want to report missing dates. If so, then you may replace what I aliased as t1 with a calendar table. For the sake of demonstration, you can inline all the dates:
SELECT
t1.date_of_birth,
COUNT(*) AS count
FROM
(
SELECT '1987-09-08' AS date_of_birth UNION ALL
SELECT '1987-09-09' UNION ALL
SELECT '1987-09-10' UNION ALL
SELECT '1987-09-11' UNION ALL
SELECT '1987-09-12'
) t1
LEFT JOIN yourTable t2
ON t1.date_of_birth >= t2.date_of_birth
GROUP BY
t1.date_of_birth;
Demo
In practice, your calendar table would be a bona fide table which just contains all the dates you want to appear in your result set.
One method is a correlated subquery:
select dob.date_of_birth,
(select count(*) from t where t.date_of_birth <= dob.date_of_birth) as running_count
from (select distinct date_of_birth from t) dob;
This is not particularly efficient. If your data has any size, variables are better (or window functions if you are using MySQL 8.0):
select date_of_birth,
(#x := #x + cnt) as running_count
from (select date_of_birth, count(*) as cnt
from t
group by date_of_birth
order by date_of_birth
) dob cross join
(select #x := 0) params;
Use subquery with correlation approach :
select date_of_birth, (select count(*)
from table
where date_of_birth <= t.date_of_birth
) as count
from table t
group by date_of_birth;
Let's consider a made up example
SELECT id, name, score.score FROM
someTable,
(select someTableId, count(*) as score FROM SecondTable GROUP BY someTableId) as score
WHERE score.someTableId == id
ORDER BY score.score DESC
Let's now assume that I have a backend computing my scoring, and that I would like to remove the subquery and insert my own data instead. I would like to know how to do this.
I would like to do something like (this is the question, because what's below doesn't work):
SELECT id, name, score.score FROM
someTable,
((12,324), (1, 342)) as score(id, score)
WHERE score.someTableId == id
ORDER BY score.score DESC
Here is an example of external data substitution to a subquery:
SELECT * FROM users WHERE id IN (SELECT user_id FROM posts WHERE thread_id = 12 GROUP BY user_id);
Without a subquery and with external data:
SELECT * FROM users WHERE id IN (1,2,3);
If I understood you correctly :
SELECT id, name, score.score FROM
someTable,
(SELECT 12 as someTableId,324 as score UNION ALL SELECT 1, 342 <UNION ALL....>) as score(id, score)
WHERE score.someTableId == id
ORDER BY score.score DESC
Thats the only way you can do it, it doesn't actually replace the the subquery, but it replace the select from the table and can improve performance if thats what you are looking for.
In MySQL you don't need to specify a from clause like a dummy table when you are just looking to fetch dummy data.
Other DBMS require a dummy table name (typically DUAL) but in MySQL it's rather straightforward:
SELECT 12 AS id, 324 AS score
UNION ALL SELECT 2, 65
UNION ALL SELECT 3, 598
UNION ALL SELECT 4, 244
You can use this as any other result-set.
I have a MySQL table from which I want to select:
1) Either "most occurring" value, if there is any prevailing
2) Or "average" value, if there is no most occurring value.
Example table 1:
value
1
2
3
4
All values are occurred equally, therefore I want to take AVG(`value`)
Example table 2:
value
1
2
2
3
Value 2 prevails, therefore I want to select the value 2.
What mysql query would do this?
Starting from Gordon's answer I tested and corrected the SQL query in SQL Fiddle:
SELECT IF(t4.numcnts = 1, t1.avgvalue, t2.topvalue) AS result
FROM (select avg(value) as avgvalue from test) t1
CROSS JOIN (select value as topvalue from test group by value order by count(*) desc limit 1) t2
CROSS JOIN join (select count(distinct cnt) as numcnts from
(select count(*) as cnt from test group by value) t3) t4
Here is the Fiddle with the two test tables (switch out test2 for test to see the result when a particular value prevails): http://sqlfiddle.com/#!2/76914/3
My changes were to use an IF instead of a CASEstatement in the SELECTclause and to add the necessary table aliases for the subselects.
The following approach calculates both values and then chooses between them:
select (case when numcnts = 1 then avgvalue else topvalue end)
from (select avg(value) as avgvalue from t) cross join
(select value as topvalue from t group by value order by count(*) desc limit 1) cross join
(select count(distinct cnt) as numcnts from (select count(*) as cnt from t group by value))
Note: if you have ties for the top, but other values as well, then an arbitrary value is returned. You don't specify what to do in this case.
Also, the SQL is untested, so it might have syntax errors.