In-depth understanding of MySQL advanced drifting (4)

Join

Cross join

The simplest type of join is a cross join, which involves multiplying tables to create a All content products.
Example:
select * from student,person;//The result is a combination of the two tables
All columns of the two tables are combined to produce a result set containing all possible combinations. This type is called a cross join, and the number of rows in the resulting table after the join is the same multiplied by the number of rows in each table used for the join.
Cross-connections like the above will have a great impact on the performance of the database server, so it is a good choice to use the where clause in the connection to filter out some records.

Inner join/outer join

Inner join is the most common type of join, and it is the most uniform, because it requires matching of every table that makes up every part of the join, and unmatched Rows will be excluded from the final result set
The most common example of an inner join is an equal join, that is, a field in the joined table is the same as that in each table. In this case, the final result set only contains rows matching the specified fields in the joined tables.

Outer joins are asymmetric, all rows on one side of the join are included in the final record, regardless of whether they match rows on the other side of the join
Depending on which side of the join is retained, sql Defines left outer join and right outer join. In a left outer join, all records from the table on the left side of the join that match the where clause will appear in the final result set. In a right outer join, all records from the table on the right side of the join that match the where clause will appear in the final result set.

Left outer join: explained as "select all rows from the left part of the connection, and for each selected row, either a matching value is displayed from the right part, or a null row is displayed." This type of join is called a left join, or left outer join. Right join or right outer join is just the opposite.

Self-join

This type of connection connects a table to itself. It is usually used to retrieve records in the table that contain inner joins to each other.

In addition to connections, MySQL 4.0 and higher also support the union operator, which is used to join the output of multiple select queries into a single result set. In most cases, this operator is used to add the result set produced by a query into different tables, creating a single table containing all results.

Use the union operator to combine as many select queries as possible, but two basic conditions must be met:
First, the number of fields returned by each select query must be the same.
Secondly, the data types of each select query field must match each other.

The union operator will automatically eliminate duplicate rows from the combined result set!

Subquery

where/having query

MySQL can contain subqueries in the where clause or having clause. Subqueries enclosed in parentheses have higher precedence than comparison and logical operators, the in operator, or the exists operator.
You can also use a subquery before a comparison operator in a having clause to filter the groups created by the parent query.

in/exists/from query

Comparison operators only apply if the result column returned by the subquery contains a value. If the result set returned by a subquery is a list of values, the comparison operator must be replaced by the in operator.

The in operator can detect whether a specific value exists in the result set. If the detection is successful, the external query will be executed.
The exists operator can be used to query whether a subquery has produced results, which allows the external query to be executed only when the exists test returns true.
The subquery can also be placed in the from clause of a select statement. The results are produced as a table.

So, the result table generated by the inner query is used in the from clause of the outer query. Such a table becomes an export table in sql.

It should be noted that when using the results of a subquery in this way, the result table generated by the inner query must first take another table name, otherwise MySQL does not know how to refer to the columns in it.

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