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This article mainly introduces what the execution order of sql statements is, which has a certain reference value. Interested friends can refer to it. I hope you will gain a lot after reading this article.

Sql statement execution order: 1, the first execution of from tab;2, where statement is to limit the conditions; 3, grouping statement [group by... Having]; 4, aggregate function; 5, select statement; 6, order by sort statement.

The execution order of sql statements:

1 、 from

Determine which table to fetch data from first, so from tab is executed first. There is a multi-table join, from tab1,tab2. Aliases can be added to the table to facilitate subsequent references.

2 、 where

The where statement qualifies the condition, and if there is no need to qualify, it is written as where 1, indicating that it is always true, with no conditions attached.

3. Group by. Having

Grouping statements, such as grouping by employee name, the fields to be grouped by row must appear in the select, otherwise an error will be reported. Having is used in conjunction with group by to qualify. Here is an example.

4. Aggregate function

The common aggregate functions are max,min and count,sum. The execution of aggregate functions comes after group by and before having. If you write aggregate functions in where, you will make an error.

5. Select statement

Select the fields you want to find, and if you select all, you can select *. The name of the employee and the total salary for all months are selected here.

6 、 order by

Sort statement, which defaults to ascending sort. If you want to sort in descending order, write order by [XX] desc. The order by statement is executed at the end, and can be sorted only if the select selects the fields to look for.

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