Describes how to implement multi-table database connection query and how to implement database connection query.

Describes how to implement multi-table database connection query and how to implement database connection query.

Detailed description of how to implement multi-table database connection Query

You can use the join operator to query multiple tables. Connection is the main feature of the relational database model and a symbol that distinguishes it from other types of database management systems.

In the relational database management system, the relationship between data does not have to be determined when a table is created, and all information about an object is often stored in a table. When retrieving data, you can use the join operation to query information about different entities in multiple tables. Connection operations bring great flexibility to users. They can add new data types at any time. Create new tables for different entities and then query them through connections.

The connection can be established in the FROM clause or WHERE clause of the SELECT statement. It is similar that it helps to distinguish the connection operation FROM the search conditions in the WHERE clause when the clause points out the connection. Therefore, this method is recommended in Transact-SQL.

The connection syntax format for the FROM clause defined by the SQL-92 standard is:

FROM join_table join_type join_table [ON (join_condition)] 

Join_table indicates the name of the table involved in the join operation. The join operation can be performed on the same table or on multiple tables. The Join Operation on the same table is also called a self-Join Operation.

Join_type indicates the connection type, which can be divided into three types: internal connection, external connection, and cross connection. Inner join uses a comparison operator to compare data in some columns of a table, and lists the data rows in these tables that match the connection conditions. According to the comparison method used, internal connections are classified into equivalent connections, natural connections, and unequal connections.

Outer join is divided into three types: left outer join (left outer join or left join), right outer join (right outer join or right join), and full outer join (full outer join or full join. Different from internal connections, external connections not only list the rows that match the connection conditions, but also list the left table (when the left Outer Join is performed) and the right table (when the right outer join is performed) or all data rows that meet the search criteria in two tables (when the table is fully connected.

Cross join does not have a WHERE clause. It returns the Cartesian product of all data rows in the JOIN table, the number of rows in the result set is equal to the number of rows that meet the query conditions in the first table multiplied by the number of rows that meet the query conditions in the second table.

The ON (join_condition) clause in the Join Operation specifies the join condition, which consists of columns, comparison operators, and logical operators in the connected table.
No matter which connection is used, you cannot directly connect the columns of the text, ntext, and image data types, but you can indirectly connect these columns.

(1) inner connection

The inner join query operation lists the data rows that match the connection condition. It uses the comparison operator to compare the column values of the connected columns. Intranet connections are divided into three types:

1. equijoin: Use the equal sign (=) operator in the connection condition to compare the column values of connected columns. All columns in the connected table, including duplicate columns, are listed in the query results.

2. Unequal join: Use a comparison operator other than the equal operator to compare the column values of the connected columns. These operators include >,>=, <=, <,!> ,! <And <>.

3. Natural join: Use the equal to (=) operator in the connection condition to compare the column values in the connected column. However, it uses the selection list to indicate the columns included in the query result set, delete duplicate columns in the connection table.

For example, the following uses equijoin to list authors and publishers in the same city in the authors and publishers tables:

SELECT * FROM authors AS a INNER JOIN publishers AS p ON a.city=p.city 

If you use a natural connection, delete the duplicate columns (city and state) in the authors and publishers tables in the selection list ):

SELECT a.*,p.pub_id,p.pub_name,p.country FROM authors AS a INNER JOIN publishers AS p ON a.city=p.city

(2) External Connection

Only the rows that meet the query conditions (WHERE search conditions or HAVING conditions) and connection conditions in the returned query result set are returned. When an external connection is used, it returns to the query result set that contains not only rows that meet the connection conditions, but also the left table (when the left outer connection is used) and the right table (when the right outer connection is used) or all data rows in two edge join tables (all Outer Join.

Outer Join can be left Outer Join, right outer join, or complete external join.

When an external JOIN is specified in the FROM clause, it can be specified by one of the following sets of keywords: left join or left outer join; right join or right outer join; full join or full outer join.

(1) left outer Join: The result set of the left outer Join includes all rows in the LEFT table specified in the left outer clause, not just the rows matched by the join column. If a row in the left table does not match a row in the right table, all selection list columns in the right table in the row of the associated result set are null.

(2) Right outer join: Right outer join is a reverse join of left Outer Join. All rows in the right table are returned. If a row in the right table does not match a row in the left table, a null value is returned for the left table.

(3) Complete External join: The Complete External join returns all rows in the left and right tables. If a row does not match a row in another table, the selection list column of the other table contains a null value. If there are matched rows between tables, the entire result set row contains the data value of the base table.

Rows are returned only when at least one row in the same two tables meets the join conditions. The inner join removes rows that do not match any row in the other table. The outer join will return all rows of at least one table or view mentioned in the FROM clause, as long as these rows meet any WHERE or HAVING search conditions. Searches all rows in the left table referenced by the left Outer Join and all rows in the right table referenced by the right outer join. All rows of the two tables in the complete external join will be returned.

For example, use the left outer link to connect the Forum content to the author information:

SELECT a.*,b.* FROM luntan LEFT JOIN usertable as b ON a.username=b.username 

Next we will use a full outer join to list all the authors in the city table, all the authors in the user table, and their cities:

SELECT a.*,b.* FROM city as a FULL OUTER JOIN user as b ON a.username=b.username

(3) cross join

A crossover clause without a WHERE clause returns the Cartesian product of all data rows in the two joined tables, the number of rows returned to the result set is equal to the number of rows that meet the query conditions in the first table multiplied by the number of rows that meet the query conditions in the second table.

For example, if there are 6 types of books in the titles table, and there are 8 publishers in the publishers table, the number of records retrieved by the following cross join will be 6*8 = 48 rows.

SELECT type,pub_name FROM titles CROSS JOIN publishers ORDER BY type

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