SQL Syntax Reference Manual (SQL)

zhaozj2021-02-11  190

SQL Syntax Reference Manual (SQL)

DB2 provides a query language SQL (Structure Query Language) of the off-site database, which is a very spoken, easy-to-understand syntax. This language is almost per database system must be provided to indicate a connection operation, including data definition (DDL), and processing processing (DML). SQL originally metrics, the prototype of this language is completed in the IBM San Jose laboratory with the name "System R". It is quite satisfactory, and the result is quite satisfactory and determines in the system. R The technical foundation of R is developed from IBM products. Moreover, the National Standard Society (ANSI) and the International Standardization Organization (ISO) follows an almost IBM SQL-based standard-based standard-off language definition in 1987.

First, the data definition DDL (Data Definition Language)

Data-oriented language refers to the language defined in the format and morphology of the information. He is the first to face when each database is to be established. Which of the formats are all in the form, what kind of key key, form, and The relationship between the tables, etc., all must plan at the beginning.

1, build a form:

Create Table Table_name

Column1 DataType [NOT NULL] [NOT NULL PRIMARY KEY],

Column2 Datatype [NOT NULL],

...)

Description:

DataType - is the format of the data, see the table.

NUT NULL - You can not allow information to be available (not yet filled in).

Primary Key - is the primary key of this table.

2, change the form

Alter Table Table_name

Add Column Column_name DataType

Description: Add a field (no grammar to delete a certain field.

Alter Table Table_name

Add Primary Key (Column_Name)

Description: Change the definition of an analog to set a certain field as a primary key.

Alter Table Table_name

Drop Primary Key (Column_Name)

Description: Delete the definition of the primary key.

3, establish an index

Create index index_name on table_name (Column_name)

Description: Establish an index for a table's field to increase the speed of the query.

4, delete

Drop Table_name

Drop Index_name

Second, the data form DATATYPES

Smallint

The integer of 16-bit yuan.

Interger

32-bit integers.

Decimal (p, s)

p Precision value and the size of the size of the S size, the precise value P refers to all of the number (DIGITS) large value, S is a decimal

There are several digits after the point. If there is no special designation, the system is set to P = 5; s = 0.

Float

32-bit elements.

Double

The real number of 64-bit yuan.

char (n)

N the string of the length, n cannot exceed 254.

VARCHAR (N)

The length of the length is not fixed and its maximum length is N, N cannot exceed 4000.

Graphic (n)

Like char (n), but its unit is two characters Double-Bytes, n cannot exceed 127. This form is to support two fonts of the length of the two characters, such as Chinese characters.

Vargraphic (n)

The double-character string of variable length and its maximum length is N, N cannot exceed 2000.

date

Contains year, month, date.

Time

It contains hours, minutes, and seconds.

Timestamp

Includes year, month, day, hour, minute, second, one thousandth.

Third, data operation DML (Data Manipulation Language)

After the data is defined, the next thing is the operation of the information. The operation is not more than an Insert, query information (Query), change the information (UPDATE), deleting four modes, the following points introduce their syntax:

1, increase information:

INSERT INTO TABLE_NAME (Column1, Column2, ...)

Values ​​(Value1, Value2, ...)

Description:

1. If you do not specify a column system, you will be filled in the order in the field within the table.

2. The data form of the field and the information filled in the field must be consistent.

3.Table_name can also be a landscape view_name.

INSERT INTO TABLE_NAME (Column1, Column2, ...)

Select Columnx, Columny, ... from another_table

Explanation: You can also fill in other forms of tables through a subquery.

2, inquiry information:

Basic query

Select Column1, Columns2, ...

From table_name

Explanation: All of the specific fields of Table_name is listed all.

SELECT *

From table_name

Where column1 = xxx

[And color] [or column3 <> zzz]

Description:

1. '*' Indicates that all fields are listed.

2.Where is the conditional pattern, and the eligible information is listed.

Select Column1, Column2

From table_name

Order by column2 [desc]

Description: Order by is specified to be sorted in a certain field, [dec] means from big to small, if there is no indication, it is from small to large

arrangement

Combination query

Combined query refers to the source of information that is queried does not only have a single form, but a combination of more than one.

Table can be obtained.

SELECT *

From Table1, Table2

Where table1.colum1 = Table2.column1

Description:

1. Query the same information on the same value in both tables.

2. Of course, the two forms are compared to each other, and their data must be the same.

3. A complex query that it can be used to use may be many.

Integration query:

SELECT Count (*)

From table_name

Where colorn_name = xxx

Description:

There are a few of the qualified information.

SELECT SUM (Column1)

From table_name

Description:

1. Calculate the sum, the selected field must be a number of digital forms.

2. Avg () is the integrated query for calculating average, max () and min () calculate the maximum minimum.

Select Column1, AVG (Column2)

From table_namegroup by column1

Having Avg (Column2)> XXX

Description:

1.Group by: Keywords that calculate the average of COLUMN2 with column1 for a set of column2

use together.

2.Having: It must be used as a constraint for integration with Group By.

Complex query

SELECT *

From table_name1

WHERE EXISTS

SELECT *

From table_name2

WHERE CONDitions

Description:

1.where's Conditions can be another Query.

2.Exists herein refers to whether there is.

SELECT *

From table_name1

WHERE column1 in

Select Column1

From table_name2

WHERE CONDitions

Description:

1. The later is connected is a collection, indicating that the column1 exists in a collection.

2. SELECT's data must meet Column1.

Other query

SELECT *

From table_name1

Where colorn1 like 'x%'

Note: The elements of LIKE must be in the back 'X%' indicating the string starting with X.

SELECT *

From table_name1

WHERE column1 in ('xxx', 'yyy', ..)

Note: The later is connected is a collection, indicating that the column1 exists in a collection.

SELECT *

From table_name1

Where column1 betWeen XX and YY

Note: Between indicates that the value of Column1 is between XX and YY.

3. Change information:

Update Table_name

Set column1 = 'xxx'

WHERE CONDITOINS

Description:

1. Change a certain field setting It is 'xxx'.

2. Conditions is the condition you have to match. If there is no WHERE, all TABLE will be all changed.

4, delete information:

Delete from table_name

WHERE CONDitions

Description: Delete eligible information.

Note: About WHERE conditions If there is a date, different databases have different expressions. details as follows:

(1) If it is an Access database, it is: where mydate> # 2000-01-01 #

(2) If it is an Oracle database, it is: where mydate> cast ('2000-01-01' as Date)

Or: where mydate> to_date ('2000-01-01', 'YYYY-MM-DD')

Write in Delphi:

TheDate = '2000-01-01';

Query1.sql.add ('Select * from abc where mydate> Cast (' '' '' theDate '' ' ' as date) ');

If the comparative date is:

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