Detailed description of the SQL Server database link query method, sqlserver

Detailed description of the SQL Server database link query method, sqlserver

The knowledge about the SQL Server database link query method is what we will introduce in this article. We know that multiple tables can be queried through the connection operator. Connection is the main feature of the relational database model and a symbol that distinguishes it from other types of database management systems. Multi-table join query is the basic operation of SQL, but there are many connection methods. Skilled use of these connection methods can simplify SQL statements and improve database operation efficiency.

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 a new table for different entities and then query it through the connection.

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.

Basic syntax and Methods

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. The comparison method is different.

1. Inner Connections are classified into equivalent connections, natural connections, and unequal connections.

2. 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.

3. 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. For example:

SELECT p1.pub_id,p2.pub_id,p1.pr_info   FROM pub_info AS p1 INNER JOIN pub_info AS p2   ON DATALENGTH(p1.pr_info)=DATALENGTH(p2.pr_info) 

(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  SELECT a.*,p.pub_id,p.pub_name,p.country  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 ):

(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.

1. left join or left outer join (left outer join)

Left Outer Join: (also called left Outer Join. The matching rows of the two tables are given, and the table on the left prevails. If there are rows in the left table but not in the right table, the column selected in the corresponding row of the right table is displayed as NULL, and multiple matching records in the right table are allowed in the baseline table on the left.) The left join is the matching row on the left, check whether the table on the right has rows.

Select field1, field2 from table1 left join table2 on field1 = field2 (reference field, which can be multiple) where table1.field3 = table2.field3

2. right join or right outer join (right outer join)

Right join: (this is also called the right outer join. The matching rows of the two tables are given, and the table on the right prevails. If the table on the right has rows that do not exist in the table on the left, the column selected in the corresponding row of the right table is displayed as NULL, and multiple matching records in the left table are allowed in the baseline table on the right)

The syntax is similar to the link!

(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 

This article describes how to query SQL Server database links!