My SQL Interview Questions and Answers

How MySQL Optimizes DISTINCT ?
DISTINCT is converted to a GROUP BY on all columns, DISTINCT combined with ORDER BY will in many cases also need a temporary table.

When combining LIMIT # with DISTINCT, MySQL will stop as soon as it finds # unique rows.

If you don't use columns from all used tables, MySQL will stop the scanning of the not used tables as soon as it has found the first match.

SELECT DISTINCT t1.a FROM t1,t2 where t1.a=t2.a;

In the case, assuming t1 is used before t2 (check with EXPLAIN), then MySQL will stop reading from t2 (for that particular row in t1) when the first row in t2 is found.

How MySQL Optimizes LEFT JOIN and RIGHT JOIN ?
A LEFT JOIN B in MySQL is implemented as follows:

The table B is set to be dependent on table A and all tables that A is dependent on.
The table A is set to be dependent on all tables (except B) that are used in the LEFT JOIN condition.
All LEFT JOIN conditions are moved to the WHERE clause.
All standard join optimizations are done, with the exception that a table is always read after all tables it is dependent on. If there is a circular dependence then MySQL will issue an error.
All standard WHERE optimizations are done.
If there is a row in A that matches the WHERE clause, but there wasn't any row in B that matched the LEFT JOIN condition, then an extra B row is generated with all columns set to NULL.
If you use LEFT JOIN to find rows that don't exist in some table and you have the following test: column_name IS NULL in the WHERE part, where column_name is a column that is declared as NOT NULL, then MySQL will stop searching after more rows (for a particular key combination) after it has found one row that matches the LEFT JOIN condition.
RIGHT JOIN is implemented analogously as LEFT JOIN.

The table read order forced by LEFT JOIN and STRAIGHT JOIN will help the join optimizer (which calculates in which order tables should be joined) to do its work much more quickly, as there are fewer table permutations to check.

Note that the above means that if you do a query of type:

SELECT * FROM a,b LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key) WHERE b.key=d.key

MySQL will do a full scan on b as the LEFT JOIN will force it to be read before d.

The fix in this case is to change the query to:

SELECT * FROM b,a LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key) WHERE b.key=d.key

How MySQL Optimizes LIMIT ?
In some cases MySQL will handle the query differently when you are using LIMIT # and not using HAVING:

If you are selecting only a few rows with LIMIT, MySQL will use indexes in some cases when it normally would prefer to do a full table scan.
If you use LIMIT # with ORDER BY, MySQL will end the sorting as soon as it has found the first # lines instead of sorting the whole table.
When combining LIMIT # with DISTINCT, MySQL will stop as soon as it finds # unique rows.
In some cases a GROUP BY can be resolved by reading the key in order (or do a sort on the key) and then calculate summaries until the key value changes. In this case LIMIT # will not calculate any unnecessary GROUP BY's.
As soon as MySQL has sent the first # rows to the client, it will abort the query.
LIMIT 0 will always quickly return an empty set. This is useful to check the query and to get the column types of the result columns.
The size of temporary tables uses the LIMIT # to calculate how much space is needed to resolve the query.

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