从MySQL 5开始, 你可以看到多了一个系统数据库information_schema . information_schema 存贮了其他所有数据库的信息。让我们来看看几个使用这个数据库的例子：
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1. 取得关于 information_schema的基本信息
information_schema是一个虚拟数据库，并不物理存在，在select的时候，从其他数据库获取相应的信息。
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| bugs |
| mysql |
| sugarcrm |
+--------------------+
4 rows in set (0.00 sec)
以下是information_schema数据库中的表.
mysql> use information_schema;
mysql> show tables;
+---------------------------------------+
| Tables_in_information_schema |
+---------------------------------------+
| CHARACTER_SETS |
| COLLATIONS |
| COLLATION_CHARACTER_SET_APPLICABILITY |
| COLUMNS |
| COLUMN_PRIVILEGES |
| KEY_COLUMN_USAGE |
| PROFILING |
| ROUTINES |
| SCHEMATA |
| SCHEMA_PRIVILEGES |
| STATISTICS |
| TABLES |
| TABLE_CONSTRAINTS |
| TABLE_PRIVILEGES |
| TRIGGERS |
| USER_PRIVILEGES |
| VIEWS |
+---------------------------------------+
17 rows in set (0.00 sec)
2. 查询表中数据超过1000行的表
以下的语句可以查出超过1000行数据的表
mysql> select concat(table_schema,'.',table_name) as table_name,table_rows
-> from information_schema.tables where table_rows > 1000
-> order by table_rows desc;
+----------------------------------+------------+
| table_name | table_rows |
+----------------------------------+------------+
| bugs.series_data | 52778 |
| bugs.bugs_activity | 26436 |
| bugs.longdescs | 21473 |
| bugs.email_setting | 5370 |
| bugs.attachments | 4714 |
| bugs.attach_data | 4651 |
| bugs.cc | 4031 |
| bugs.bugs | 2190 |
| bugs.namedqueries_link_in_footer | 1228 |
+----------------------------------+------------+
9 rows in set (0.04 sec)
3. 查询所有没有主键的表
This example gives a list of all the tables without primary key.
SELECT CONCAT(t.table_name,".",t.table_schema) as table_name
FROM information_schema.TABLES t
LEFT JOIN information_schema.TABLE_CONSTRAINTS tc
ON t.table_schema = tc.table_schema
AND t.table_name = tc.table_name
AND tc.constraint_type = 'PRIMARY KEY'
WHERE tc.constraint_name IS NULL
AND t.table_type = 'BASE TABLE';
4. 实现表的历史数据information_schema
Putting the MySQL information_schema to Use article implements a history database using the information schema. The first half of this article describes the requirements for the history database, and a generic design to implement it. The second half describes the stepwise construction of code-generator that creates the SQL to construct and load the history database. The code-generator is driven by the information schema and some features of the information schema are discussed in detail.
5. 查询5个最大表
mysql> SELECT concat(table_schema,'.',table_name) table_name,
-> concat(round(data_length/(1024*1024),2),'M') data_length
-> FROM information_schema.TABLES
-> ORDER BY data_length DESC LIMIT 5;
+--------------------+-------------+
| table_name | data_length |
+--------------------+-------------+
| bugs.attach_data | 706.89M |
| bugs.longdescs | 3.45M |
| bugs.bugs_activity | 1.45M |
| bugs.series_data | 0.75M |
| bugs.attachments | 0.51M |
+--------------------+-------------+
5 rows in set (0.05 sec)
--转自
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2014-11-27 9:50:31 |
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