Master SQL four most basic data manipulation statements

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Master SQL four most basic data manipulation statements: insert,select,update and delete.

Mastering SQL is a valuable asset for database users. In this article, we will guide you through the core functions of the four most basic data manipulation statements-sql-to introduce the comparison operator, the selection assertion, and the three-valued logic in turn. After you have completed these studies, you are clearly beginning to be proficient in SQL.

Before we start, use the CREATE TABLE statement to make a table (as shown in Figure 1). DDL statements define database objects such as tables, columns, and vision. They do not process rows in a table because DDL statements do not handle the actual data in the database. The work is handled by another type of SQL statement-Data Manipulation Language (DML) statement.

There are four basic DML operations in SQL: Insert,select,update and delete. Since this is a frequent use of most SQL users, it is necessary to have a one by one description of them here. In Figure 1 we give a table named employees. Each of these lines corresponds to a specific employee record. Please familiarize yourself with this form, which we will use later in the example.

Connection Query

Multiple table queries can be implemented by connecting operators. Connection is the main feature of relational database model, and it is also a symbol distinguishing from other types of database management system.

In the relational database management system, the relationship between the data in the table is not determined, and all the information of an entity is stored in a table. When retrieving data, queries the information of different entities that are stored in multiple tables through a JOIN operation. Connection operations give users a lot of flexibility, and they can add new data types at any time. Create a new table for different entities, and then query by connection.

A connection can be established in the FROM clause or a WHERE clause of a SELECT statement, and paradoxically, in the FROM clause, it helps to differentiate the join operation from the search conditions in the WHERE clause. Therefore, the use of 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)]

Where join_table indicates the name of the table participating in the JOIN operation, the connection can operate on the same table, or a multiple table operation, and the connection to the same table operation is called a self connection.

Join_type indicates the type of connection, which can be divided into three types: inner, outer, and Cross joins. The INNER join (inner join) uses the comparison operator to compare operations of some (some) column data between tables and lists the rows of data in those tables that match the join condition. According to the comparison method used, the inner connection is divided into equivalent connection, natural connection and unequal connection three kinds.

An outer join is divided into three kinds of left outer joins (either the right outer join or the Ieft join), the right-hand outer join (the outer join, or the OK join), and the full outer join (either a fully outer join or a complete join). Unlike an inner connection, the outer join lists not only the rows that match the join condition, but all the rows of data that match the search criteria in the left table (when left outer), right (when the right outer join), or two tables (when all outer joins).

A cross join (cross join) does not have a WHERE clause that returns the Cartesian product of all rows of data in the join table, with the number of rows in the result set equal to the number of rows in the first table that match the query criteria multiplied by the number of rows of data in the second table that match the query criteria

The ON (join_condition) clause in a JOIN operation indicates the join condition, which consists of columns and comparison operators, logical operators, and so on in the connected table.
No connection can be directly connected to the text, ntext, and image data type columns, but the three types of columns may be indirectly connected. 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)

(i) Internal connections

The INNER JOIN query operation lists the rows of data that match the join criteria, which compares the column values of the connected columns using comparison operators. The inner connection is divided into three kinds:

1. Equivalent connection: Use the equals sign (=) operator in the join condition to compare the column values of the connected columns, whose query results list all the columns in the connected table, including the repeating columns.

2. Unequal connection: The column values of the concatenated columns are compared using comparison operators other than the equals operator in the join condition. These operators include >, >=, <=, <,!>,!<, and <>.

3. Natural connection: Use the Equals (=) operator in the join condition to compare the column values of the connected columns, but it uses a select list to indicate the columns included in the query result collection, and deletes the duplicate columns from the attached table.

For example, the following uses an equivalent connection 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

Also, if you are using a natural connection, delete the repeating columns (city and state) in the authors and Publishers tables in the select 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

(ii) OUTER joins

In the query results collection, only rows that meet the query criteria (where search conditions or having conditions) and join conditions are returned. When an outer join is used, it returns to the query result collection not only for rows that meet the join criteria, but also for all data rows in the left table (when the left outer join), right table (when the right outer join), or two side tables (full outer joins).

Connect Forum content and author information as follows using the left OUTER join:

Select a.*,b.* from Luntan left join Usertable as B

On A.username=b.username

The following uses an Out-of-band connection to all authors in the city table and all authors in the user table, as well as the cities in which they reside:

Select a.*,b.*

From city as a full outer join user as B

On A.username=b.username

(iii) Cross-linking

A cross join does not take a WHERE clause, which returns the Cartesian product of all rows of data in the two tables that are connected, and the number of rows returned to the result set equals the number of rows in the first table that match the query criteria multiplied by the number of rows in the second table that match the query criteria.

For example, there are 6 categories of books in the titles table, and there are 8 publishers in the publishers table, the number of records retrieved by the following cross joins will wait

On the 6*8=48 line.

Select Type,pub_name

From titles Cross join publishers

ORDER BY Type

The Union operator can combine a collection of query results from two or more SELECT statements into a single result set, that is, to execute a federated query. The syntax format of the Union is:

Select_statement

Union [all] Selectstatement

[Union [all] selectstatement] [... n]

Where Selectstatement is the SELECT query statement to be joined.

The all option means that all rows are merged into the result collection. When this item is not specified, the duplicate rows in the Federated query result collection are persisted to only one row.

In a federated query, the column headings of the query results are the column headings of the first query statement. Therefore, to define a column heading must be defined in the first query statement. To sort federated query results, you must also use the column name, column heading, or column ordinal in the first query statement.

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