Oracle教程 2023-02-24

Oracle 视图 DBA_TAB_COLUMNS 官方解释，作用，如何使用详细说明

本站中文解释

Oracle视图DBA_TAB_COLUMNS用于显示所有表（包括表的结构和数据类型）的字段信息，也就是说，可以使用这个视图来显示某一张表的所有列信息，也可以显示某一个列的信息。

可以使用以下语句来查询所有表信息：

SELECT * FROM DBA_TAB_COLUMNS;

可以使用以下语句来查询某一张表中的所有列信息, 其中table_name参数表示要查询的表名：

SELECT * FROM DBA_TAB_COLUMNS WHERE table_name= table_name ;

可以使用以下语句来查询某一张表中某一个指定列的信息，其中table_name参数表示要查询的表名，column_name参数表示要查询的列名：

SELECT * FROM DBA_TAB_COLUMNS WHERE table_name= table_name AND column_name = column_name ;

官方英文解释

DBA_TAB_COLUMNS describes the columns of all tables, views, and clusters in the database.

Its columns (except for SENSITIVE_COLUMN) are the same as those in ALL_TAB_COLUMNS.

To gather statistics for this view, use the DBMS_STATS package.

This view filters out system-generated hidden columns. The DBA_TAB_COLS view does not filter out system-generated hidden columns.

Columns marked with an asterisk (*) in the table below remain for backward compatibility with Oracle7. This information is now in the [TAB|PART]_COL_STATISTICS views.

Column	Datatype	NULL	Description
`OWNER`	`VARCHAR2(128)`	`NOT NULL`	Owner of the table, view, or cluster
`TABLE_NAME`	`VARCHAR2(128)`	`NOT NULL`	Name of the table, view, or cluster
`COLUMN_NAME`	`VARCHAR2(128)`	`NOT NULL`	Column name
`DATA_TYPE`	`VARCHAR2(128)`		Data type of the column
`DATA_TYPE_MOD`	`VARCHAR2(3)`		Data type modifier of the column
`DATA_TYPE_OWNER`	`VARCHAR2(128)`		Owner of the data type of the column
`DATA_LENGTH`	`NUMBER`	`NOT NULL`	Length of the column (in bytes)
`DATA_PRECISION`	`NUMBER`		Decimal precision for `NUMBER` data type; binary precision for `FLOAT` data type; NULL for all other data types
`DATA_SCALE`	`NUMBER`		Digits to the right of the decimal point in a number
`NULLABLE`	`VARCHAR2(1)`		Indicates whether a column allows NULLs. The value is `N` if there is a `NOT NULL` constraint on the column or if the column is part of a `PRIMARY KEY.` The constraint should be in an `ENABLE VALIDATE` state.
`COLUMN_ID`	`NUMBER`		Sequence number of the column as created
`DEFAULT_LENGTH`	`NUMBER`		Length of the default value for the column
`DATA_DEFAULT`	`LONG`		Default value for the column
`NUM_DISTINCT*`	`NUMBER`		Number of distinct values in the column
`LOW_VALUE*`	`RAW(1000)`		Low value in the column
`HIGH_VALUE*`	`RAW(1000)`		High value in the column
`DENSITY*`	`NUMBER`		If a histogram is available on `COLUMN_NAME`, then this column displays the selectivity of a value that spans fewer than 2 endpoints in the histogram. It does not represent the selectivity of values that span 2 or more endpoints. If a histogram is not available on `COLUMN_NAME`, then the value of this column is `1`/`NUM_DISTINCT`.
`NUM_NULLS`	`NUMBER`		Number of NULLs in the column
`NUM_BUCKETS`	`NUMBER`		Number of buckets in the histogram for the column Note: The number of buckets in a histogram is specified in the `SIZE` parameter of the `ANALYZE` SQL statement. However, Oracle Database does not create a histogram with more buckets than the number of rows in the sample. Also, if the sample contains any values that are very repetitious, Oracle Database creates the specified number of buckets, but the value indicated by this column may be smaller because of an internal compression algorithm.
`LAST_ANALYZED`	`DATE`		Date on which this column was most recently analyzed
`SAMPLE_SIZE`	`NUMBER`		Sample size used in analyzing this column
`CHARACTER_SET_NAME`	`VARCHAR2(44)`		Name of the character set: `CHAR_CS` `NCHAR_CS`
`CHAR_COL_DECL_LENGTH`	`NUMBER`		Declaration length of the character type column
`GLOBAL_STATS`	`VARCHAR2(3)`		`GLOBAL_STATS` will be `YES` if statistics are gathered or incrementally maintained, otherwise it will be `NO`
`USER_STATS`	`VARCHAR2(3)`		Indicates whether statistics were entered directly by the user (`YES`) or not (`NO`)
`AVG_COL_LEN`	`NUMBER`		Average length of the column (in bytes)
`CHAR_LENGTH`	`NUMBER`		Displays the length of the column in characters. This value only applies to the following data types: `CHAR` `VARCHAR2` `NCHAR` `NVARCHAR2`
`CHAR_USED`	`VARCHAR2(1)`		Indicates that the column uses `BYTE` length semantics (`B`) or `CHAR` length semantics (`C`), or whether the data type is not any of the following (NULL): `CHAR` `VARCHAR2` `NCHAR` `NVARCHAR2`
`V80_FMT_IMAGE`	`VARCHAR2(3)`		Indicates whether the column data is in release 8.0 image format (`YES`) or not (`NO`)
`DATA_UPGRADED`	`VARCHAR2(3)`		Indicates whether the column data has been upgraded to the latest type version format (`YES`) or not (`NO`)
`HISTOGRAM`	`VARCHAR2(15)`		Indicates existence/type of histogram: `NONE` `FREQUENCY` `TOP-FREQUENCY` `HEIGHT BALANCED` `HYBRID`
`DEFAULT_ON_NULL`	`VARCHAR2(3)`		Indicates whether the column has DEFAULT ON NULL semantics (`YES`) or not (`NO`)
`IDENTITY_COLUMN`	`VARCHAR2(3)`		Indicates whether this is an identity column (`YES`) or not (`NO`)
`SENSITIVE_COLUMN`	`VARCHAR2(3)`		Indicates whether this is a sensitive column (`YES`) or not (`NO`)
`EVALUATION_EDITION`	`VARCHAR2(128)`		Name of the edition in which editioned objects referenced in an expression column are resolved
`UNUSABLE_BEFORE`	`VARCHAR2(128)`		Name of the oldest edition in which the index may be used as part of a query plan
`UNUSABLE_BEGINNING`	`VARCHAR2(128)`		Name of the edition for which the index may not be used as part of a query plan in this edition or any of its descendants
`COLLATION`	`VARCHAR2(100)`		Collation for the column. Only applies to columns with character data types.

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