SQL入门-DQL数据查询语言--select

Posted 2020-10-19 anyux

1.help select

mysql> help select 
Name: \'SELECT\'
Description:
Syntax:
SELECT
    [ALL | DISTINCT | DISTINCTROW ]
      [HIGH_PRIORITY]
      [STRAIGHT_JOIN]
      [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
      [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
    select_expr [, select_expr ...]
    [FROM table_references
      [PARTITION partition_list]
    [WHERE where_condition]
    [GROUP BY {col_name | expr | position}
      [ASC | DESC], ... [WITH ROLLUP]]
    [HAVING where_condition]
    [ORDER BY {col_name | expr | position}
      [ASC | DESC], ...]
    [LIMIT {[offset,] row_count | row_count OFFSET offset}]
    [PROCEDURE procedure_name(argument_list)]
    [INTO OUTFILE \'file_name\'
        [CHARACTER SET charset_name]
        export_options
      | INTO DUMPFILE \'file_name\'
      | INTO var_name [, var_name]]
    [FOR UPDATE | LOCK IN SHARE MODE]]

SELECT is used to retrieve rows selected from one or more tables, and
can include UNION statements and subqueries. See [HELP UNION], and
http://dev.mysql.com/doc/refman/5.6/en/subqueries.html.

The most commonly used clauses of SELECT statements are these:

o Each select_expr indicates a column that you want to retrieve. There
  must be at least one select_expr.

o table_references indicates the table or tables from which to retrieve
  rows. Its syntax is described in [HELP JOIN].

o Starting in MySQL 5.6.2, SELECT supports explicit partition selection
  using the PARTITION keyword with a list of partitions or
  subpartitions (or both) following the name of the table in a
  table_reference (see [HELP JOIN]). In this case, rows are selected
  only from the partitions listed, and any other partitions of the
  table are ignored. For more information and examples, see
  http://dev.mysql.com/doc/refman/5.6/en/partitioning-selection.html.

  In MySQL 5.6.6 and later, SELECT ... PARTITION from tables using
  storage engines such as MyISAM that perform table-level locks (and
  thus partition locks) lock only the partitions or subpartitions named
  by the PARTITION option.

  See
  http://dev.mysql.com/doc/refman/5.6/en/partitioning-limitations-locki
  ng.html, for more information.

o The WHERE clause, if given, indicates the condition or conditions
  that rows must satisfy to be selected. where_condition is an
  expression that evaluates to true for each row to be selected. The
  statement selects all rows if there is no WHERE clause.

  In the WHERE expression, you can use any of the functions and
  operators that MySQL supports, except for aggregate (summary)
  functions. See
  http://dev.mysql.com/doc/refman/5.6/en/expressions.html, and
  http://dev.mysql.com/doc/refman/5.6/en/functions.html.

SELECT can also be used to retrieve rows computed without reference to
any table.

URL: http://dev.mysql.com/doc/refman/5.6/en/select.html

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2.select 可以不使用from 子句

select user();

3.指定查询条件,使用数据过滤的更加准确

select user,host,password from mysql.user;

 

 4.模糊查询

select user,host,password from mysql.user where user like \'roo%\';

 5.排序order by

--降序排列
select * from anyuxweb.t2 order by id desc;

--升序排列
select * from anyuxweb.t2 order by id asc;

 

 6.limit 使用

select * from anyuxweb.t2 order by id asc limit 1,2;