单个查询用于多个sql查询

Question

I am working on a project where a user can add comments and also hit any post.

Now I have to display the total number of comments and total number of hits and also show whether the user has already hitted that post or not.

So basically I need to do three sql queries for this action:

• one counting comments,
• one for counting hits and
• one for checking whether the user has hitted the post or not.

I wanted to know that if it's possible to reduce these three sql queries into one and reduce the database load?

Any help is appreciated.

$checkifrated=mysql_query("select id from fk_views where (onid='$postid' and hit='hit' and email='$email')");//counting hits
        $checkiffollowing=mysql_query("select id from fk_views where (onid='$postid' and hit='hit' and email='$email')");
 $hitcheck=mysql_num_rows($checkifrated);//checking if already hited or not

$checkifrated=mysql_query("select id from fk_views where (onid='$postid' and comment !='' and email='$email')");//counting comments

Answer 1

This query returns the number of hits and number of nonempty comments.

select ifnull(sum(hit='hit'),0) as hits, ifnull(sum(comment !=''),0) as comments 
  from fk_views where onid='$postid' and email='$email'

Based on the queries you provided I dont think you need to query separately if he is hitted the post, just check in you code if number of hits is > 0

Answer 2

Yes, it may be possible to combine the three queries into a single query. That may (or may not) "reduce the database load". The key here is going to be an efficient execution plan, which is going to primarily depend on the availability of suitable indexes.

Combining three inefficient queries into one isn't going to magically make the query more efficient. The key is getting each of the queries to be as efficient as they can be.

If each of the queries is processing rows from the same table, then it may be possible to have a single SELECT statement process the entire set, to obtain the specified result. But if each of the the queries is referencing a different table, then it's likely the most efficient would be to combine them with a UNION ALL set operator.

Absent the schema definition, the queries that you are currently using, and the EXPLAIN output of each query, it's not practical to attempt to provide you with usable advice.

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UPDATE

Based on the update to the question, providing sample queries... we note that two of the queries appear to be identical.

It would be much more efficient to have a query return a COUNT() aggregate, than pulling back all of the individual rows to the client and counting them on the client, eg

SELECT COUNT(1) AS count_hits
  FROM fk_views v
 WHERE v.onid  = '42'
   AND v.hit   = 'hit'
   AND v.email = 'someone@email.address'

To combine processing of the three queries, we can use conditional expressions in the SELECT list. For example, we could use the equality predicates on the onid and email columms in the WHERE clause, and do the check of the hit column with an expression... For example:

SELECT SUM(IF(v.hit='hit',1,0)) AS count_hits 
     , SUM(1)                   AS count_all
  FROM fk_views v
 WHERE v.onid = '42'
   AND v.email='someone@email.address'

The "trick" to getting three separate queries combined would be to use a common set of equality predicates (the parts of the WHERE clause that match in all three queries).

SELECT SUM(IF(v.hit='hit'  ,1,0)) AS count_hits 
     , SUM(IF(v.comment!='',1,0)) AS count_comments
     , SUM(1)                     AS count_all
  FROM fk_views v
 WHERE v.onid = '42'
   AND v.email ='someone@email.address'

If we are going to insist on using the deprecated mysql interface (over PDO or mysqli) it's important that we use the mysql_real_escape_string function to avoid SQL Injection vulnerabilities

$sql = "SELECT SUM(IF(v.hit='hit'  ,1,0)) AS count_hits
             , SUM(IF(v.comment!='',1,0)) AS count_comments
             , SUM(1)                     AS count_all
          FROM fk_views v
         WHERE v.onid = '" . mysql_real_escape_string($postid) . "'
          AND v.email = '" . mysql_real_escape_string($email) ;

# for debugging
#echo $sql

$result=mysql_query($sql);
if (!$result) die(mysql_error());

while ($row = mysql_fetch_assoc($result)) {
    echo $row['count_hits'];
    echo $row['count_comments'];
}

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For performance, we'd likely want an index with leading columns of onid and email , eg

... ON fk_views (onid,email)

The output from EXPLAIN will show the execution plan.