Data | Database Author: scholar
At the beginning of this article, I would like to thank some friends for their letters to me pointing out the mistakes in the previous articles. I do not know if you remember in the eighth article I gave a simple use of
Application
Produced page access counters? A friend of mine asked me why when he changed the counter start variable
NumVisits
Value has no effect on the final count result? At first I was puzzled, let's recall the source program as follows:
<%
Dim NumVisits
Numvisits=0
Application.Lock
Application ("NumVisits") = Application ("NumVisits") + 1
Application.UnLock
%>
Welcome to this page, you are the first <%= application ("NumVisits")
%> Guest!
In this program, if you try to change the variable numvisits
Value to change the result of the register, it is absolutely impossible. Because the value of the variable is used to change
Application
The value is not possible, and the two are irrelevant. Therefore, the definition and assignment of variables are superfluous. So how exactly are we supposed to give
Application ("NumVisits")
Define an initial value? Please see the following revised procedure:
<%
If application ("NumVisits") < 999 Then
Application ("NumVisits") =999
End If
Application.Lock
Application ("NumVisits") =application ("NumVisits") +1
Application.UnLock
%>
Welcome to this page, you are the first <%= application ("NumVisits")
%> Guest!
Here's 999
is the initial value of the counter you want to set, so the problem is solved. I am very grateful to the friend surnamed Kang for pointing out this mistake to me, although this is only a very small loophole, but we in the process of common program to write procedures, we really need this kind of rigorous and meticulous style, hope that in the future when friends find the text of the error in the letter immediately informed, I can also timely correction, thank you.
Sql
The language can be divided into two parts: data definition language and data manipulation language, and then we learned the language of data manipulation
After the Select statement, today's author will continue to give you a brief description of the remaining SQL statements.
Sql
is a kind of complete data processing language, not only for database query, but also for data modification and update in database, and support
The complexity of the Select statement of the SQL query compares the SQL of the database content changes
Statement is exceptionally simple. However, for a DBMS
, the risk of data updates is far beyond the data query. The database management system must protect the consistency of the stored data during the change period, ensure the effective data enters the database, and the database must be consistent.
Dbms
You must also coordinate concurrent updates for multiple users to ensure that users and their changes do not affect the jobs of other users.
The following three SQL statements are used to modify the contents of a database:
1, insert, add new data rows to a table
2, delete, remove data rows from a table
3, Update, change the data already existing in the database
First let's look at the use of Insert:
Standard syntax:
INSERT into table_name
(col1, col2 ...)
VALUES (value1, value2 ...)
The following example will be the scholar as a new salesman into the table Salesreps
Insert into
Salesreps (Name,num,sales,hire_date,income)
VALUES (' Shusheng ', 9,10000, ' 23-feb-99 ', 2000)
In this statement, the name of the column is separated by commas in parentheses, followed by the Value
For each column of data that is also separated by commas in the phrase and parentheses, it should be noted that the order of the data and column names is the same, and if the string is separated by single quotes. Conceptually speaking,
An Insert statement establishes a row of data that is consistent with the table column structure, taken from Values
clause, and then add the new row to the table, and the rows in the table are not sorted, so there is no such notion of inserting the row between the head or tail of the table or the two rows.
After the Insert statement is finished, the new row is part of the table.
The Insert statement can also add multiple rows of data to the target table, in this form
Insert
statement, the data value of the new row is not specified explicitly in the body of the statement, but is a database query specified in the statement. It may seem strange to add values from the rows of the database itself, but this is useful in some specific state. For example, do you want to
Order number, date, and number of orders generated before December 30, 1998
Table to another table named Oldorder, multiple rows Insert
Statement provides a compact and efficient way to copy data, as follows:
Insert into Oldorder (Num,date,amount)
Select Num,date,amount
From order
Where date< ' 30-12-98 '
This statement looks a bit complicated, but it's simple, the statement identifies the table that receives the new row
Oldorder and receiving data columns, exactly like a single row Insert
Statement. The remainder of the statement is a query that retrieves the data in the order table. Sql
Execute a query on the order table, and then insert the query results line by row into the Oldorder
Table.
Let's look at the use of update, update
Statement is used to update the values of one or more columns of a selected row in a single table. The target table to be updated is defined in the statement.
The SET clause specifies which columns to update and calculates their values. The Update statement always contains
Where statement, and the Update statement is more dangerous, you must explicitly recognize where
Statement, where statements are used to specify the rows that need to be updated.
Standard syntax:
UPDATE table_name
SET columnname1 = value1
[, columname2 = value2] ...
WHERE search_condition
The following is an example of a simple Update statement:
Update Customers
Set credit=100000.00,id=99
Where name= ' ASP '
In this example, we will table customers the customer name ASP
The customer's credit value is updated to 100,000 and his ID is changed to 99. Then look at the following example:
Update Customers
Set credit=200000.00,state=021
Where ID in (80,90,100,120)
We can find that in fact SQL processing Update
The process of a statement is to iterate through the specified table, to update the rows whose search criteria result is true, and to skip rows whose search criteria result is "false" or "empty."
Finally, take a look at the Delete statement.
Standard syntax:
DELETE from TableName
WHERE condition
Because it is too simple, so the consequences are serious, although the
The WHERE clause is optional, but it almost always exists, if the WHERE clause is from the Delete
statement, all rows of the target table are deleted. Look at the following example:
Delete from Order Where id=99
