MySQL默认值和约束的查询方法

2022-06-28 10:03 浏览：214

1. 列出 MySQL 数据库中的表默认值

select table_schema as database_name,
       table_name,
       column_name,
       column_default
from information_schema.columns
where  column_default is not null
      and table_schema not in ('information_schema', 'sys',
                               'performance_schema','mysql')
--    and table_schema = 'your database name'
order by table_schema,
         table_name,
         ordinal_position;

说明：

database_name - 数据库的名称（模式）
table_name - 表的名称
column_name - 列的名称
column_default - 定义此列默认值的 SQL 表达式

2. MySQL数据库默认值汇总

select column_default,
       count(distinct concat(col.table_schema, '.', col.table_name)) as tables,
       count(column_name) as columns
from information_schema.columns col
join information_schema.tables tab on col.table_schema = tab.table_schema
                                   and col.table_name = tab.table_name
where col.table_schema not in ('sys', 'information_schema', 
                               'mysql', 'performance_schema')
      and tab.table_type = 'base TABLE'
group by column_default
order by tables desc,
         columns desc;

说明：

column_default -为没有默认值的列定义默认约束（公式）和NULL
tables- 具有此约束的表数（或具有对NULL行没有约束的列的表数）
columns - 具有此特定约束的列数（或NULL行没有约束）

二、约束

1. 列出了数据库（模式）中的所有不可为空的列

select tab.table_schema as database_name,
    tab.table_name,
    col.ordinal_position as column_id,
    col.column_name,
    col.data_type,
    case when col.numeric_precision is not null
            then col.numeric_precision
        else col.character_maximum_length end as max_length,
    case when col.datetime_precision is not null
            then col.datetime_precision
        when col.numeric_scale is not null
            then col.numeric_scale
        else 0 end as 'precision'
from information_schema.tables as tab
join information_schema.columns as col
        on col.table_schema = tab.table_schema
        and col.table_name = tab.table_name
        and col.is_nullable = 'no'
where tab.table_schema not in ('information_schema', 'sys', 
                               'mysql','performance_schema')
      and tab.table_type = 'base TABLE'
      -- and tab.table_schema = 'database name'
order by tab.table_schema,
         tab.table_name,
         col.ordinal_position;

注意：如果您需要特定数据库（模式）的信息，请取消注释 where 子句中的条件并提供您的数据库名称。

说明：

database_name - 数据库的名称（模式）
table_name - 表的名称
column_id - 列在表中的位置
column_name - 列的名称
data_type - 列数据类型
max_length - 数据类型的最大长度
precision- 数据类型的精度

2. 检查 MySQL 数据库中的列是否可为空

select c.table_schema as database_name,
       c.table_name,
       c.column_name,
       case c.is_nullable
            when 'NO' then 'not nullable'
            when 'YES' then 'is nullable'
       end as nullable
from information_schema.columns c
join information_schema.tables t
     on c.table_schema = t.table_schema 
     and c.table_name = t.table_name
where c.table_schema not in ('mysql', 'sys', 'information_schema',
                             'performance_schema')
      and t.table_type = 'base TABLE'
      -- and t.table_schema = 'database_name' -- put your database name here
order by t.table_schema,
         t.table_name,
         c.column_name;