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This article mainly explains "what are the skills of using SOL". Friends who are interested may wish to have a look. The method introduced in this paper is simple, fast and practical. Next, let the editor take you to learn "what are the skills for using SOL?"

Routines-select (results to be obtained) + from (data source) + where (qualified conditions) + order by (which item is ranked)

The most important thing: draw their diagrammatic process correctly so that you can see it at a glance. Be sure to draw a diagram! )

1.distinct hire_date-- enables employees to be grouped according to the date (that is, the same data is put in a group) to achieve the purpose of de-duplication. (group by can also be weighed and more efficient.)

two。 It is divided into sorting from small to big (asc) and sorting from big to small (desc). The default is asc sort.

3.limit 2mae3Mel-start the query with the data with index 2 and return the results of the first three queries

4. Primary key-only one record can be found by this key. When there is more than one record, it cannot be set as the primary key. In addition, you can set the two keys as primary keys together.

5. Foreign key-if the primary key in table An is the key in table B, it can be said that there is a foreign key in table A, thus establishing the relationship between the two tables

6.having function: similar to where, the difference is where filter row, having filter group. Order of use: where, having, group by (avoid duplicate data), order by, limit

7. Join: a left join b, then an is the main table (left join), a right join b, then b is the main table. The best illustration example-see here (right link) for details. There is also (internal connection) here.

Right join-that is, all the data of the main table is taken out, and then the secondary table is joined with the relevant items of the main table according to the conditions (that is, the schedule should match the main table)

At this point, I believe you have a deeper understanding of "what are the skills for the use of SOL?" you might as well do it in practice. Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

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