Step-by-Step learning of complex Oracle queries (key points)

Preface complex data statistics are often executed in practical applications and data from multiple tables needs to be displayed. Therefore, we must pay great attention to the complex select statement! 1-important functions of Data grouping: max, min, avg, sum, count (1

Preface complex data statistics are often executed in practical applications and data from multiple tables needs to be displayed. Therefore, we must pay great attention to the complex select statement! 1-important functions of Data grouping: max, min, avg, sum, count (1

Preface

Complex data statistics are often required in practical applications, and data in multiple tables must be displayed.

Example:

If there are no 40 departments, change to right join... ON, as shown in:

We can see from the above: 40 departments have appeared!

All databases outside of Oracle (SQLSERVER, MYSQL, DB2, and so on) use the preceding SQL: 1999 syntax operation, so it must be! (Of course, if you keep using the Oracle database, you won't :))

Emphasize that the performance of Multi-table queries is certainly not high, and the performance must be discovered only when the data volume is large.

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