MySQL :: MySQL 8.4 参考手册 :: 28.3.16 INFORMATION_SCHEMA KEY_COLUMN

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MySQL 8.4 参考手册 / 28 个 INFORMATION_SCHEMA 表 / 28.3 INFORMATION_SCHEMA 总表 / 28.3.16 INFORMATION_SCHEMA KEY_COLUMN_USAGE 表

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28.3.16 INFORMATION_SCHEMA KEY_COLUMN_USAGE 表28.3.16 The INFORMATION_SCHEMA KEY_COLUMN_USAGE Table

KEY_COLUMN_USAGE 表描述了哪些键列具有约束。该表不提供关于函数键部分的信息，因为它们是表达式，而该表只提供关于列的信息。The KEY_COLUMN_USAGE table describes which key columns have constraints. This table provides no information about functional key parts because they are expressions and the table provides information only about columns.

KEY_COLUMN_USAGE 表具有以下列：The KEY_COLUMN_USAGE table has these columns:

CONSTRAINT_CATALOGCONSTRAINT_CATALOG
约束所属的目录名称。该值始终为 def。The name of the catalog to which the constraint belongs. This value is always def.
CONSTRAINT_SCHEMACONSTRAINT_SCHEMA
约束所属的模式（数据库）名称。The name of the schema (database) to which the constraint belongs.
CONSTRAINT_NAMECONSTRAINT_NAME
约束的名称。The name of the constraint.
TABLE_CATALOG
表所属的目录名称。该值始终为 def。The name of the catalog to which the table belongs. This value is always def.
TABLE_SCHEMA
表所属的模式（数据库）名称。The name of the schema (database) to which the table belongs.
TABLE_NAMETABLE_NAME
具有约束的表的名称。The name of the table that has the constraint.
COLUMN_NAMECOLUMN_NAME
具有约束的列的名称。The name of the column that has the constraint.
如果约束是外键，那么这是外键的列，而不是外键引用的列。If the constraint is a foreign key, then this is the column of the foreign key, not the column that the foreign key references.
ORDINAL_POSITION
列在约束中的位置，而不是列在表中的位置。列的位置编号从 1 开始。The column's position within the constraint, not the column's position within the table. Column positions are numbered beginning with 1.
POSITION_IN_UNIQUE_CONSTRAINT
对于唯一约束和主键约束为 NULL。对于外键约束，此列是被引用表中键的序号位置。NULL for unique and primary-key constraints. For foreign-key constraints, this column is the ordinal position in key of the table that is being referenced.
REFERENCED_TABLE_SCHEMA
约束引用的架构名称。The name of the schema referenced by the constraint.
REFERENCED_TABLE_NAME
约束引用的表名称。The name of the table referenced by the constraint.
REFERENCED_COLUMN_NAME
约束引用的列名称。The name of the column referenced by the constraint.

假设有两个表名为 t1 和 t3，它们具有以下定义：Suppose that there are two tables name t1 and t3 that have the following definitions:

CREATE TABLE t1
(
    s1 INT,
    s2 INT,
    s3 INT,
    PRIMARY KEY(s3)
) ENGINE=InnoDB;

CREATE TABLE t3
(
    s1 INT,
    s2 INT,
    s3 INT,
    KEY(s1),
    CONSTRAINT CO FOREIGN KEY (s2) REFERENCES t1(s3)
) ENGINE=InnoDB;

对于这两个表，KEY_COLUMN_USAGE 表有两行记录：For those two tables, the KEY_COLUMN_USAGE table has two rows:

一行记录的 CONSTRAINT_NAME = 'PRIMARY'，TABLE_NAME = 't1'，COLUMN_NAME = 's3'，ORDINAL_POSITION = 1，POSITION_IN_UNIQUE_CONSTRAINT = NULL。One row with CONSTRAINT_NAME = 'PRIMARY', TABLE_NAME = 't1', COLUMN_NAME = 's3', ORDINAL_POSITION = 1, POSITION_IN_UNIQUE_CONSTRAINT = NULL.
对于 NDB：此值始终为 NULL。For NDB: This value is always NULL.
一行，CONSTRAINT_NAME = 'CO'，TABLE_NAME = 't3'，COLUMN_NAME = 's2'，ORDINAL_POSITION = 1，POSITION_IN_UNIQUE_CONSTRAINT = 1。One row with CONSTRAINT_NAME = 'CO', TABLE_NAME = 't3', COLUMN_NAME = 's2', ORDINAL_POSITION = 1, POSITION_IN_UNIQUE_CONSTRAINT = 1.

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