SQL language query basics: Connection query joint query code

SQL language query basics: 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:
Inner join: Compares (some) column data between tables using comparison operators, 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: it can be 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: no 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
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 connected tables (all Outer Join.
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 crossover will be
Rows 6*8 = 48.
Select type, pub_name
FROM titles cross join publishers
OrDER BY type
SQL language query basics: Joint Query
The UNION operator can combine the query result sets of two or more Select statements into one result set for display, that is, the UNION query is executed. The syntax format of UNION is:
Select_statement
UNION [ALL] selectstatement
[UNION [ALL] selectstatement] [… N]
Selectstatement is the Select query statement to be combined.
The ALL option combines ALL rows into the result set. If this parameter is not specified, only one row is retained for the duplicate row in the Union query result set.
During a joint query, the column title of the query result is the column title of the first query statement. Therefore, to define a column title, it must be defined in the first query statement. To sort the Union query results, you must also use the column name, column title, or column number in the first query statement.
When using the UNION operator, ensure that there are the same number of expressions in the selection list of each joint query statement, and each query selection expression should have the same data type, or they can be automatically converted to the same data type. During automatic conversion, the system converts low-precision data types to high-precision data types.
In UNION statements that contain multiple queries, the execution sequence is from left to right. Brackets can be used to change the execution sequence. For example:
Query 1 UNION (query 2 UNION query 3)