mysql in query repeat

In MySQL, we often use "in" queries to find results in a list of values. However, if there are duplicate values ​​in the list of values, we may get unexpected results when using the "in" query. Therefore, when using "in" queries, it is necessary for us to understand the impact of duplicate values ​​and master methods to avoid duplicate values.

Why do duplicate values ​​affect the "in" query? Let's illustrate with an example. Suppose we have a table called "students" that contains student ID, name, age, and class fields. We now have a query requirement to find out all students in classes 1, 2, and 3. The statement using "in" query is as follows:

SELECT * FROM students WHERE class IN (1,2,3);

The above statement can meet the needs well, but if we include repeated values ​​in the class list, for example:

SELECT * FROM students WHERE class IN (1,2,3,2);

At this time, the query results will include students in Class 2 appearing twice. This repeated result will not only affect the accuracy of the query results, but also reduce the query efficiency and increase the system load.

So, how to avoid this situation? We can use a keyword called "distinct" to remove duplicate values. The query statement using the "distinct" keyword is as follows:

SELECT DISTINCT * FROM students WHERE class IN (1,2,3,2);

In this way, students in class 2 will not appear repeatedly in the query results.

In addition to using the "distinct" keyword to avoid duplicate values, we can also avoid this situation by removing duplicate values ​​in the value list. For example:

SELECT * FROM students WHERE class IN (1,2,3) OR class=2;

The above statement is equivalent to the previously mentioned statement, but it removes duplicate values ​​in the class list.

It should be noted that using "IN" may cause performance problems when processing value lists containing a large number of duplicate values. At this time, we can consider using "JOIN" query, "EXISTS" query or temporary table instead of "in" query. The use of these methods can effectively improve query efficiency and avoid the impact of duplicate values.

In short, when using "in" query, we should pay attention to the existence of duplicate values ​​and its possible impact. Avoiding the occurrence of duplicate values ​​can improve the accuracy and efficiency of queries and ensure the stability of the system.

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