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MySQL - SELECT WHERE field IN (subquery) - Extremely slow why?
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Rewrite the query into this
SELECT st1.*, st2.relevant_field FROM sometable st1
INNER JOIN sometable st2 ON (st1.relevant_field = st2.relevant_field)
GROUP BY st1.id /* list a unique sometable field here*/
HAVING COUNT(*) > 1
I think st2.relevant_...
What is the “N+1 selects problem” in ORM (Object-Relational Mapping)?
The "N+1 selects problem" is generally stated as a problem in Object-Relational mapping (ORM) discussions, and I understand that it has something to do with having to make a lot of database queries for something that seems simple in the object world.
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MySQL select 10 random rows from 600K rows fast
How can I best write a query that selects 10 rows randomly from a total of 600k?
26 Answers
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In Ruby, is there an Array method that combines 'select' and 'map'?
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I usually use map and compact together along with my selection criteria as a postfix if. compact gets rid of the nils.
jruby-1.5.0 > [1,1,1,2,3,4].map{|n| n*3 if n==1}
=> [3, 3, 3, nil, nil, nil]
jruby-1.5.0 > [1,1,1,2,3,4].map{|n| n*3 if n==1}.compact
=> ...
Generate a random number in the range 1 - 10
... 1 and 10 you mean any float that is >= 1 and < 10, then it's easy:
select random() * 9 + 1
This can be easily tested with:
# select min(i), max(i) from (
select random() * 9 + 1 as i from generate_series(1,1000000)
) q;
min | max
-----------------+------------------...
What does it mean by select 1 from table?
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SELECT 1 FROM TABLE_NAME means, "Return 1 from the table". It is pretty unremarkable on its own, so normally it will be used with WHERE and often EXISTS (as @gbn notes, this is not necessarily best practice, it is, however, c...
MySQL: Insert record if not exists in table
...oDB;
Insert a record:
INSERT INTO table_listnames (name, address, tele)
SELECT * FROM (SELECT 'Rupert', 'Somewhere', '022') AS tmp
WHERE NOT EXISTS (
SELECT name FROM table_listnames WHERE name = 'Rupert'
) LIMIT 1;
Query OK, 1 row affected (0.00 sec)
Records: 1 Duplicates: 0 Warnings: 0
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Real life example, when to use OUTER / CROSS APPLY in SQL
...1) Top N per group queries (can be more efficient for some cardinalities)
SELECT pr.name,
pa.name
FROM sys.procedures pr
OUTER APPLY (SELECT TOP 2 *
FROM sys.parameters pa
WHERE pa.object_id = pr.object_id
ORDER BY pr.n...
Difference between Select Unique and Select Distinct
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SELECT UNIQUE is old syntax supported by Oracle's flavor of SQL. It is synonymous with SELECT DISTINCT.
Use SELECT DISTINCT because this is standard SQL, and SELECT UNIQUE is non-standard, and in database brands other than ...
How to get multiple counts with one SQL query?
...tion. This is basically the same thing as a PIVOT function in some RDBMS:
SELECT distributor_id,
count(*) AS total,
sum(case when level = 'exec' then 1 else 0 end) AS ExecCount,
sum(case when level = 'personal' then 1 else 0 end) AS PersonalCount
FROM yourtable
GROUP BY distributor_id
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