我可以根据结果数量将MySQL查询重构为一个查询吗？

Question

I have a table y Which has two columns a and b

Entries are:

a   b

1   2

1   3

1   4

0   5

0   2

0   4

I want to get 2,3,4 if I search column a for 1, and 5,2,4 if I search column a .

So, if I search A for something that is in A, (1) I get those rows, and if there are no entries A for given value, give me the 'Defaults' ( a = '0')

Here is how I would know how to do it:

$r = mysql_query('SELECT `b` FROM `y` WHERE `a` = \'1\';');
//This gives desired results, 3 rows

$r = mysql_query('SELECT `b` FROM `y` WHERE `a` = \'2\';');
//This does not give desired results yet.
//Get the number of rows, and then get the 'defaults'
if(mysql_num_rows($r) === 0) $r = mysql_query('SELECT `b` FROM `y` WHERE `a` = 0;');

So, now that it's sufficiently explained, how do I do that in one query, and what about performance concerns?

The most used portion would be the third query, because there would only be values in a for a number IF you stray from the defaults.

Answer 1

You can try something like this. I'm not 100% sure it will work because count() is a aggregate function but its worth a shot.

SELECT b
FROM table1 
WHERE a = (
   SELECT
     CASE count(b)
       WHEN 0 THEN :default_value
       ELSE :passed_value 
     END
   FROM table1
   WHERE a = :passed_value
)

Answer 2

I think I have it:

SELECT b FROM y where a=if(@value IN (select a from y group by a),@value,0);

It checks if @value exists in the table, if not, then it uses 0 as a default. @value can be a php value too.

Hope it helps :)

Answer 3

What about

$rows = $db->fetchAll('select a, b FROM y WHERE a IN (2, 0) ORDER BY a DESC');
if(count($rows) > 0) {
  $a = $rows[0]['a'];
  $i = 0;
  while($rows[$i]['a'] === $a) {
    echo $rows[$i++]['b']."\n";
  }
}

One query, but overhead if there are a lot of 'zero' values.
Depends if you care about the overhead...

Answer 4

I think Michal Kralik best answer in my opinion based on server performance. Doing subselects or stored procedures for such simple logic really is not worth it.

The only way I would improve on Michal's logic is if you are doing this query multiple times in one script. In this case I would query for the 0's first, and then run each individual query, then checking if there was any value.

Pseudo-code

// get the value for hte zero's
$zeros = $db->fetchAll('select a, b FROM y WHERE a = 0');

//checking for 1's
$ones = $db->fetchAll('select a, b FROM y WHERE a = 1');
if(empty($ones)) $ones = $zeros;

//checking for 2's
$twos = $db->fetchAll('select a, b FROM y WHERE a = 2');
if(empty($twos)) $twos = $zeros;

//checking for 3's
$threes = $db->fetchAll('select a, b FROM y WHERE a = 3');
if(empty($threes)) $threes = $zeros;

Answer 5

You can do all this in a single stored procedure with a single parameter.

I have to run out, but I'll try to write one up for you and add it here as soon as I get back from my errand.

Answer 6

I don't know why this was marked down - please educate me. It is a valid, tested stored procedure, and I answered the question. The OP didn't require that the answer be in php. ??

Here's a stored proc to do what you want that works in SQL Server. I'm not sure about MySQL.

create proc GetRealElseGetDefault (@key as int)
as
begin

-- Use this default if the correct data is not found
declare @default int
select @default = 0

-- See if the desired data exists, and if so, get it.  
-- Otherwise, get defaults.
if exists (select * from TableY where a = @key)
    select b from TableY where a = @key
else
    select b from TableY where a = @default

end -- GetRealElseGetDefault

You would run this (in sql server) with

GetRealElseGetDefault 1

Based on a quick google search, exists is fast in MySQL. It would be especially fast is column A is indexed. If your table is large enough for you to be worried about performance, it is probably large enough to index.