16の異なるクエリ例を使用したMySQLのWHERE条件

反対のケースはMySQLWHERE NOT IN 句。これは、クエリが条件に指定された値を含む行を無視することを確認するために使用されます。上記の例では、ヨーロッパの4か国からの顧客をリクエストしました。 MySQLのWHERE NOT IN SELECT 同じリクエストをクエリすると、前に定義した4つを除き、他の国に居住するすべての顧客が返されます。

SELECT ContactName, Country, City
FROM Customers
WHERE Country NOT IN ('Belgium', 'France', 'Germany', 'UK')
ORDER BY Country;

出力：

注 ：[NOT] EXISTSの使用をお勧めします [NOT] IN以上 EXISTS以降 論理演算子はINよりも高速に動作します 、サブクエリを使用してデータセットを比較する場合。さらに、NOT EXISTS ソースデータにNULLが含まれている場合、予期しない結果を回避できます 値。

WHEREISNULL演算子

値がNULLであるかどうかを確認するには かどうかにかかわらず、IS NULLを使用します オペレーター。次のステートメントは、WHEREを使用しています IS NULLを含む句 books_readに値のない行を取得する演算子 列：

SELECT last_name, first_name, reg_year, books_read
FROM readers
WHERE books_read IS NULL;

出力は次のようになります：

WHERE ISNOTNULLの例

前のコマンドとは異なり、WHERE IS NOT NULL 逆に、問題の列にある種の値を含む行のみが表示されます。次の例では、 books_readが存在する行のみを選択します。 列が空ではありません：

SELECT last_name, first_name, reg_year, books_read
FROM readers
WHERE books_read IS NOT NULL;

出力には、 books_readの値を含む行のみが表示されます。 列：

WHERE EXISTS

EXISTS operator checks if the subquery returns any value at all. Typically, this operator is used to indicate that at least one row in a table satisfies a certain condition.

In the example below,

SELECT last_name, first_name
FROM customers
WHERE EXISTS 
(SELECT * FROM discounts WHERE customers.id = discounts.customer_id)

WHERE NOT EXISTS

However, the NOT EXISTS operator is there for us in case we want to make sure whether there are rows in the table that do not satisfy a certain condition. For example, let’s find all products from the Products table for which there were no orders in the Orders table:

SELECT last_name, first_name
FROM customers
WHERE NOT EXISTS 
(SELECT * FROM discounts WHERE customers.id = discounts.customer_id)

There is a separate article in our blog for those willing to learn more about the difference between EXISTS and NOT EXISTS in SQL.

Using MySQL WHERE clause with comparison operators

As we know, the WHERE condition value can be any number, text, string, or subquery. Besides specifying the condition strictly, we can set a wider meaning for it by using the comparison operators. One example of a similar function is using the BETWEEN operator. Now, let us review other options to set the condition range.

In our example, we want to retrieve the list of customers with a customer ID lower than 90 who reside in the USA. The query is the following:

SELECT CustomerID, ContactName, City, Country 
FROM Customers
WHERE CustomerID < 90 AND Country ='USA';

The output is:

Summary

As you see, MySQL WHERE is among the most helpful clauses for all MySQL users. The variety of different logical operators lets you focus on the specific portion of data only, filter it correctly, and receive the results you need. Besides, modern professional tools for MySQL databases allow you to perform all tasks faster and easier. They take the burden of manual routines off your shoulders and make the performance much more stable and reliable. To sum up, working with a WHERE statement is not too complicated. However, it requires being familiar with the corresponding commands. The command-line tools are more oriented toward experienced tech-savvy users. On the contrary, dbForge Studio for MySQL can boast an intuitive graphical interface. The intelligent code completion feature will be of use while working with the WHERE clause, navigating the code, obtaining quick object information, and instantly performing syntax checks. Moreover, dbForge Query Builder will be of use to easily build WHERE clauses visually.

Useful Links

• Joins in MySQL
• MySQL update query