SQL function learning (coalesce of functions in actual projects)

2013/01/25

1. Obtain the first non-empty value. (Practical application of functions in Projects)

Coalesce (A, B, C) is the first time I have used this function in a project.

The design requirements are as follows:

If the user "represents the user ID" is used as the query condition for subsequent operations,

Otherwise, the "user ID" is used as the query condition.

The implementation is as follows:

Coalesce (C. top_user_id, A. user_id) as "userid"

Summary:

Although this code does not look like anything,

However, if you are not familiar with SQL functions, it is easy to put your judgment into Java logic for implementation.

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Benefits:

The clever way of writing this is that the user ID you need to query later is returned at once, and you do not have to judge it in the Java logic.