Problems with NULL Values
The concept of the NULL value is a common source of confusion for newcomers to SQL, who often think that NULL is the same thing as an empty string ''. This is not the case. For example, the following statements are completely different:mysql> INSERT INTO my_table (phone) VALUES (NULL);
mysql> INSERT INTO my_table (phone) VALUES ('');
Both statements insert a value into the phone column, but the first inserts a NULL value and the second inserts an empty string. The meaning of the first can be regarded as “phone number is not known” and the meaning of the second can be regarded as “the person is known to have no phone, and thus no phone number.”To help with NULL handling, you can use the IS NULL and IS NOT NULL operators and the IFNULL() function.In SQL, the NULL value is never true in comparison to any other value, even NULL. An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for the operators and functions involved in the expression. All columns in the following example return NULL:mysql> SELECT NULL, 1+NULL, CONCAT('Invisible',NULL);
To search for column values that are NULL, you cannot use an expr = NULL test. The following statement returns no rows, because expr = NULL is never true for any expression:mysql> SELECT * FROM my_table WHERE phone = NULL;
To look for NULL values, you must use the IS NULL test. The following statements show how to find the NULL phone number and the empty phone number:mysql> SELECT * FROM my_table WHERE phone IS NULL;
mysql> SELECT * FROM my_table WHERE phone = '';
See Section 3.3.4.6, “Working with NULL Values”, for additional information and examples.You can add an index on a column that can have NULL values if you are using the MyISAM, InnoDB, or BDB, or MEMORY storage engine. Otherwise, you must declare an indexed column NOT NULL, and you cannot insert NULL into the column.When reading data with LOAD DATA INFILE, empty or missing columns are updated with ''. To load a NULL value into a column, use \N in the data file. The literal word “NULL” may also be used under some circumstances. See Section 13.2.6, “LOAD DATA INFILE Syntax”.When using DISTINCT, GROUP BY, or ORDER BY, all NULL values are regarded as equal.When using ORDER BY, NULL values are presented first, or last if you specify DESC to sort in descending order.Aggregate (summary) functions such as COUNT(), MIN(), and SUM() ignore NULL values. The exception to this is COUNT(*), which counts rows and not individual column values. For example, the following statement produces two counts. The first is a count of the number of rows in the table, and the second is a count of the number of non-NULL values in the age column:mysql> SELECT COUNT(*), COUNT(age) FROM person;
For some data types, MySQL handles NULL values specially. If you insert NULL into a TIMESTAMP column, the current date and time is inserted. If you insert NULL into an integer or floating-point column that has the AUTO_INCREMENT attribute, the next number in the sequence is inserted.

Subqueries with EXISTS or NOT EXISTS
If a subquery returns any rows at all, EXISTS subquery is TRUE, and NOT EXISTS subquery is FALSE. For example:SELECT column1 FROM t1 WHERE EXISTS (SELECT * FROM t2);
Traditionally, an EXISTS subquery starts with SELECT *, but it could begin with SELECT 5 or SELECT column1 or anything at all. MySQL ignores the SELECT list in such a subquery, so it makes no difference.For the preceding example, if t2 contains any rows, even rows with nothing but NULL values, the EXISTS condition is TRUE. This is actually an unlikely example because a [NOT] EXISTS subquery almost always contains correlations. Here are some more realistic examples:What kind of store is present in one or more cities?SELECT DISTINCT store_type FROM storesWHERE EXISTS (SELECT * FROM cities_storesWHERE cities_stores.store_type = stores.store_type);
What kind of store is present in no cities?SELECT DISTINCT store_type FROM storesWHERE NOT EXISTS (SELECT * FROM cities_storesWHERE cities_stores.store_type = stores.store_type);
What kind of store is present in all cities?SELECT DISTINCT store_type FROM stores s1WHERE NOT EXISTS (SELECT * FROM cities WHERE NOT EXISTS (SELECT * FROM cities_storesWHERE cities_stores.city = cities.cityAND cities_stores.store_type = stores.store_type));
The last example is a double-nested NOT EXISTS query. That is, it has a NOT EXISTS clause within a NOT EXISTS clause. Formally, it answers the question “does a city exist with a store that is not in Stores”? But it is easier to say that a nested NOT EXISTS answers the question “is x TRUE for all y?”