select各子句用法
1. SELECT DISTINCT:
  - This clause is used to select unique rows from a table. It eliminates duplicate rows from the result set.
2. SELECT TOP:
  - This clause is used to limit the number of rows returned by a query. It is commonly used in conjunction with the ORDER BY clause to specify the desired number of rows from the top.
3. SELECT INTO:
  - This clause is used to create a new table and insert the selected rows from an existing table into the new table. It is commonly used to create backup tables or to make copies of existing tables.
4. SELECT WHERE:
  - This clause is used to specify conditions for selecting rows from a table. It is used to filter the result set based on specified conditions.
5. SELECT FROM:
  - This clause is used to specify the table(s) from which the data is to be retrieved. It is used to identify the source table(s) for the SELECT statement.
queryselectorall用法
6. SELECT JOIN:
  - This clause is used to combine rows from two or more tables based on related columns between them. It is used to retrieve data from multiple tables by specifying the related columns to join on.
7. SELECT ORDER BY:
  - This clause is used to sort the result set in ascending or descending order based on one or more columns. It is used to arrange the selected rows in a specific order.
8. SELECT GROUP BY:
  - This clause is used to group rows based on one or more columns. It is commonly used with aggregate functions like SUM, COUNT, AVG, etc., to perform calculations on grouped rows.
9. SELECT HAVING:
  - This clause is used to filter the result set based on conditions on the grouped rows. It is similar to the WHERE clause but operates on grouped rows rather than individual rows.
10. SELECT UNION:
    - This clause is used to combine the result sets of two or more SELECT statements into a single result set. It is used to merge rows from multiple queries into a single result set.

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