SQLite中文网
The Schema Table
▼ 目录

一、简介

每个 SQLite 数据库都包含一个“模式表”，用于存储该数据库的模式。数据库的模式是对数据库中包含的所有其他表、索引、触发器和视图的描述。架构表如下所示： 

CREATE TABLE sqlite_schema(
  type text,
  name text,
  tbl_name text,
  rootpage integer,
  sql text
);

sqlite_schema 表包含模式中每个表、索引、视图和触发器（统称为“对象”）的一行，除了 sqlite_schema 表本身没有条目。有关SQLite 如何在内部使用 sqlite_schema 表的更多信息， 请参阅 文件格式文档的模式存储部分。

2.替代名称

模式表总是可以使用名称“sqlite_schema”来引用，特别是如果被模式名称限定，如“main.sqlite_schema”或“temp.sqlite_schema”。但为了历史兼容性，一些替代名称也被认可，包括：

  1. sqlite_master
  2. sqlite_temp_schema
  3. sqlite_temp_master

备选方案 (2) 和 (3) 仅适用于与每个数据库连接关联的 TEMP 数据库，但备选方案 (1) 适用于任何地方。由于历史原因，来自sqlite3_set_authorizer() 接口的回调总是使用名称 (1) 或 (3) 引用模式表。

三、Schema表的解读

schema表的字段含义如下：

type

The sqlite_schema.type column will be one of the following text strings: 'table', 'index', 'view', or 'trigger' according to the type of object defined. The 'table' string is used for both ordinary and virtual tables.

name

The sqlite_schema.name column will hold the name of the object. (UNIQUE and PRIMARY KEY constraints on tables cause SQLite to create internal indexes with names of the form "sqlite_autoindex_TABLE_N" where TABLE is replaced by the name of the table that contains the constraint and N is an integer beginning with 1 and increasing by one with each constraint seen in the table definition. In a WITHOUT ROWID table, there is no sqlite_schema entry for the PRIMARY KEY, but the "sqlite_autoindex_TABLE_N" name is set aside for the PRIMARY KEY as if the sqlite_schema entry did exist. This will affect the numbering of subsequent UNIQUE constraints. The "sqlite_autoindex_TABLE_N" name is never allocated for an INTEGER PRIMARY KEY, either in rowid tables or WITHOUT ROWID tables.

tbl_name

The sqlite_schema.tbl_name column holds the name of a table or view that the object is associated with. For a table or view, the tbl_name column is a copy of the name column. For an index, the tbl_name is the name of the table that is indexed. For a trigger, the tbl_name column stores the name of the table or view that causes the trigger to fire.

rootpage

The sqlite_schema.rootpage column stores the page number of the root b-tree page for tables and indexes. For rows that define views, triggers, and virtual tables, the rootpage column is 0 or NULL.

sql

The sqlite_schema.sql column stores SQL text that describes the object. This SQL text is a CREATE TABLE, CREATE VIRTUAL TABLE, CREATE INDEX, CREATE VIEW, or CREATE TRIGGER statement that if evaluated against the database file when it is the main database of a database connection would recreate the object. The text is usually a copy of the original statement used to create the object but with normalizations applied so that the text conforms to the following rules:

  • The CREATE, TABLE, VIEW, TRIGGER, and INDEX keywords at the beginning of the statement are converted to all upper case letters.
  • The TEMP or TEMPORARY keyword is removed if it occurs after the initial CREATE keyword.
  • Any database name qualifier that occurs prior to the name of the object being created is removed.
  • Leading spaces are removed.
  • All spaces following the first two keywords are converted into a single space.

The text in the sqlite_schema.sql column is a copy of the original CREATE statement text that created the object, except normalized as described above and as modified by subsequent ALTER TABLE statements. The sqlite_schema.sql is NULL for the internal indexes that are automatically created by UNIQUE or PRIMARY KEY constraints.

4. Schema表的创建和修改

SQLite 在创建数据库时创建模式表，并在 SQLite 用户提交 DDL 语句以供执行时修改其内容。用户一般情况下无需修改，修改后需承担数据库损坏的风险。