set null

Objective Unlike most databases, MySQL has the concept of a storage engine that can choose the optimal storage engine for different storage requirements. 1. MySQL Storage Engine Overview Plug-in storage engine is one of the

MySQL supports foreign key storage engine only InnoDB, when creating foreign keys, requires the parent table must have a corresponding index, the child table when creating foreign keys will also automatically create the corresponding index. When you

constraints:I. When creating a table, define constraintsCREATE TABLE EMP2 ( ID number (constraint) emp2_id_nn not null, name VARCHAR2 (a) not NULL, salary number (10 , 2) )

Purpose: To ensure that data conforms to some data integrity rule by setting constraints at the column or table levelImplementation: The database proactively checks the integrity of the maintenance datameans: Constraints, data types, triggers--------

InnoDB:Support transaction processing, etc.No lock ReadSupport for foreign keysSupport Row lockIndex of type Fulltext not supportedDo not save the table's exact number of rows, scan the table to calculate how many rowsDelete Table is a row of

MySQL FOREIGN key1. What is a foreign keyA specific relationship between a table and a table. Maintain the integrity and consistency of your data.User tableOrder Orders Form1.1 Delete the user, do not delete the order, the data is inconsistentInsert

the basic steps of JDBC:first, after importing Mysql-connector-java-x.x.x-bin.jar:Second, the code1. Registered driver (three ways)2. Create a Connection object (three ways)3. Create a Send command object for an SQL statement4. Execute SQL to get

Use the ALTER TABLE statement: Add Field modifying fields Modify field default values Delete a field 1. Add a field and the new field will be the last column:ALTER TABLE TableAdd (column datatype [default expr][, column

This blog mainly focuses on the creation of keys, the creation of constraints. Modify objects and delete objects.PRIMARY KEY : The primary key is a unique identifier for each row and must contain a unique value (and therefore cannot be null).

"Three major paradigms of database design"1. First normal form (1NF fromate): Each column (field) in the datasheet must be the smallest unit that cannot be split. That is to ensure the atomicity of each column.For example: UserInfo: ' Shandong

I. Paradigm in the database:Paradigm, the English name is the Normal Form, it is the British e.f.codd (relational database ancestor) in the 70 's to put forward a relational database model, the paradigm is the basis of relational database theory,

1, unsigned modified integer type, both non-negative, with this type can increase the length of data!Type size range (signed) range (unsigned) useTINYINT 1 bytes (-128,127) (0,255) Small integer valueSMALLINT 2 bytes (-32 768,32 767) (0,65 535)

Second, SQL structur Query Language Structured Query language, is the object in the Operation relational database.DDL (data definition Language), used to create or delete table operations, and to modify table constraints CREATE TABLE, ALTER TABLE,

MySQL supports foreign key storage engine only InnoDB, when creating foreign keys, requires the parent table must have a corresponding index, the child table when creating foreign keys will also automatically create the corresponding index. When you

The concept of constraints: Constraints are some of the rules that are defined in a table to maintain database integrity. You can prevent incorrect data from being inserted into a table by defining constraints for the fields in the table.Note: 1. If

Oracle built-in objects:Sequence sequence: generates an integer sequence that can often be used to populate a primary key column.To create a sequence:CREATE SEQUENCE sequence name[Start with start value ][INCREMENT by growth value ]When used: Two

1, the default way to createThe referenced data cannot be deleted when it is deleted.SQL code CREATE TABLE T_invoice (ID number () is not NULL, Invoice_no VARCHAR2 (+) not NULL, CONSTRAINT pk_invoice_id PRIMARY KEY (ID)); CREATE

By default, Nginx is a multi-process run mode. Nginx and memcached are not the same, is a multi-process mode. Maximum benefits of using multi-process mode:1. Resource independence for each process2. No need to add a variety of cumbersome

Http://www.cnblogs.com/linshui91/archive/2010/09/29/1838770.htmlI. Overview of inter-process communicationProcess communication has some of the following purposes:A, data transfer: A process needs to send its data to another process, the amount of

The role of foreign keys: maintain data consistency and integrity, primarily to control the storage of data in foreign key tables. causes the two tables to be associated, and the foreign key can only refer to the values of the columns in the outer