Comprehensive exposure to SQL syntax (1)

SQL functions and features
In fact, in the previous article, some basic functions of SQL commands have been mentioned. However, through SQL commands, programmers or database administrators (DBA) can:
(1) Create database tables . (Including setting the space that can be used by the table)
　 (2) Change the database system environment settings.
　 (3) Grant users access rights to a certain database or table.
　 (4) Create index values ​​for database tables.
　 (5) Modify the database table structure. (Create, delete or modify table fields)
　 (6) Create new data in the database.
　 (7) Delete data from the database.
　 (8) Modify data in the database.
　 (9) Query data from the database.
　These are the things that can be accomplished through SQL commands. Doesn’t it seem that there are many more functions than the word “query” represents?

　Classification of SQL syntax

In fact, there are not many SQL commands, but to use SQL to perfection, only a few commands are enough, because SQL commands are grammatical descriptions created for relational databases , so SQL plays a very powerful role in this type of database. The following will classify and introduce the basic SQL syntax commands commonly used in VB. Before explaining the commands and syntax of SQL, SQL is classified below. The types used in SQL syntax can be said to be included in these categories.

　The first category, attribute words (PRedicates)

　It is used in SQL commands to indicate the way to select the records. Such as ALL, TOP and DISTINCT, etc.

　 The second category, Declaration

　 Make declarations on the name and data type of SQL Parameter or Parameter Query, such as the declaration of PARAMETERS, etc.

　The third category, conditional clause (Clause)

　In SQL queries, some expressions are used to define the conditions of the query to narrow the scope of the search, such as WHERE.

　The fourth category, operator (Operator) and operand (Operation)

　In SQL query, it and Operation together form an expression (Expression), such as BETWEEN....AND operator and INNER JOIN operand.

　The fifth category, Function

　 Some common SQL functions, such as AVG(), are functions that find the arithmetic mean.

　 The sixth category, SQL statement (Statement)

　 The SQL statement can be said to be the main body of SQL grammar. It is used to issue instructions to a specific database and return related data. The syntax structure of SQL is basically It can be represented by the following formula: command + conditional clause

For example:

SELECT*FROM TAB WHERE TAB.NAME='A'

"FROM....WHERE" is a condition Clause, in fact, the syntax of SQL is not difficult. You only need to remember such a rule, and I believe you can quickly understand the usage of SQL.

　SQL syntax and commands


SELECT statement
SELECT[predicate]{*|table.*|[table.]field [,[table.]field2[,...]]} [AS alias1 [,alias2[ ,...]]]
FROM tableexpression [,...][IN externaldatabase]
[WHERE...]
[GROUP BY...]
[HAVING...]
[ORDER BY...]
[WITH OWNERaccess OPTION]
The SELECT statement includes the following parts
predicate



As mentioned earlier, it includes ALL, DISTINCT, DISTINCTROW, and TOP. We can use such statements to limit the results obtained after the query.

　 *

　Specify all fields from the specified table.

　table

　The name of the specified table for the fields of the selected records.

　field1,field2

　 If the field name you want to read data contains more than one field, the data will be read in the order listed.

　alias1,alias2

　 Pseudonyms used to replace the actual field names in the form.

　tableexpression

　Table name or table containing the data we want.

　Externaldatabase

　If a database other than the current one is used, define its name in externaldatabase.
　
　 ALL, DISTINCT, DISTINCTROW, TOP attribute word usage

SELECT [ALL|DISTINCT|DISTINCTROW|[TOP n[PERCENT]]] FROM table

　 ALL

If you do not specify any field data, the Microsoft Jet database The database engine will select all fields and query the required data set based on the specified conditions.

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