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The syntax structure of the select statement in SQL is: "SELECT_list [ INTO..] FROM table_name [ WHERE...] [ GROUP BY...] [ HAVING...] [ ORDER BY...]; "。

SELECT statements consist of a series of flexible clauses that together determine which data to retrieve. SELECT statements allow users to view system information for SQL Server in addition to tables and views in the general database. Before introducing the use of SELECT statements, it is necessary to introduce the basic syntax structure and execution process of SELECT statements.

Syntax structure of SELECT statement

Although the complete syntax of SELECT statements is complex, their main clauses can be summarized as follows:

SELECT select_list[ INTO new_table]FROM table_name[ WHERE search_condition][ GROUP BY group_by_expression][ HAVING search_condition][ ORDER BY order_expression [ ASC|DESC ] ]

The only clauses required are SELECT and FROM clauses; all other clauses are optional. The clauses have the following meanings:

- SELECT clause: Specifies the columns returned by the query.

- INTO clause: Stores the retrieval results in a new table or view.

- FROM clause: Used to specify the table or view in which the referenced column resides. If there is more than one object, they must be separated by commas.

- WHERE clause: Specifies search criteria to limit the rows returned. If the SELECT statement does not have a WHERE clause, the DBMS assumes that all rows in the target table satisfy the search condition.

- GROUP BY clause: Specifies the group in which the output rows are placed, and calculates the summary value for each group if the SELECT clause contains an aggregate function.

- HAVING clause: Specify search criteria for groups or aggregations. HAVING is usually used with a GROUP BY clause. Without a GROUP BY clause, HAVING behaves the same as a WHERE clause.

- ORDER BY clause: Specifies the ordering of the result set. ASC keyword indicates ascending results, DESC keyword indicates descending results. If no keyword is specified, ASC is the default keyword. Without an ORDER BY clause, the DBMS displays the data according to where it is stored in the input table.

SELECT Order and Function of Clauses

The order of clauses in a SELECT statement is important. Optional clauses can be omitted, but they must appear in the proper order when used. Their order and primary role in SELECT query statements are shown in Table 4.1.

Table 4.1 Order and effect of clauses in SELECT query statement

sequence number

clause keyword

clause function

1

SELECT

fetches data for a specified column from a specified table

2

FROM

Specify the table to query for the operation

3

WHERE

Used to specify a criteria for selecting queries

4

GROUP BY

Grouping result sets, often used with aggregate functions

5

HAVING

Returns the number of rows in the selected result set

6

ORDER BY

Specify search criteria for grouping

If some of the query clauses shown in Table 4.1 are used in the same SELECT query statement, the query clauses are arranged in descending order of their ordinal number.

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