Class jdbcResultSetMetaData
- All Implemented Interfaces:
ResultSetMetaData
,Wrapper
ResultSet
object.
The following code fragment creates the ResultSet
object rs, creates the ResultSetMetaData
object rsmd,
and uses rsmd to find out how many columns rs has and whether the
first column in rs can be used in a WHERE
clause.
ResultSet rs = stmt.executeQuery("SELECT a, b, c FROM TABLE2"); ResultSetMetaData rsmd = rs.getMetaData(); int numberOfColumns = rsmd.getColumnCount(); boolean b = rsmd.isSearchable(1);
HSQLDB-Specific Information:
HSQLDB supports a subset of the ResultSetMetaData
interface.
The JDBC specification for ResultSetMetaData
is in part very
vague. This causes potential incompatibility between interpretations of the
specification as realized in different JDBC driver implementations. As such,
deciding to what degree reporting ResultSetMetaData is accurate has been
considered very carefully. Hopefully, the design decisions made in light of
these considerations have yeilded precisely the subset of full
ResultSetMetaData support that is most commonly needed and that is most
important, while also providing, under the most common use-cases, the
fastest access with the least overhead and the best comprimise between
speed, accuracy, jar-foootprint and retention of JDBC resources.
(fredt@users)
(boucherb@users)
- Author:
- boucherb@users
- See Also:
-
Field Summary
Fields inherited from interface java.sql.ResultSetMetaData
columnNoNulls, columnNullable, columnNullableUnknown
-
Method Summary
Modifier and TypeMethodDescriptiongetCatalogName
(int column) Gets the designated column's table's catalog name.getColumnClassName
(int column) Returns the fully-qualified name of the Java class whose instances are manufactured if the methodResultSet.getObject
is called to retrieve a value from the column.int
Returns the number of columns in thisResultSet
object.int
getColumnDisplaySize
(int column) Indicates the designated column's normal maximum width in characters.getColumnLabel
(int column) Gets the designated column's suggested title for use in printouts and displays.getColumnName
(int column) Get the designated column's name.int
getColumnType
(int column) Retrieves the designated column's SQL type.getColumnTypeName
(int column) Retrieves the designated column's database-specific type name.int
getPrecision
(int column) Get the designated column's number of decimal digits.int
getScale
(int column) Gets the designated column's number of digits to right of the decimal point.getSchemaName
(int column) Get the designated column's table's schema.getTableName
(int column) Gets the designated column's table name.boolean
isAutoIncrement
(int column) Indicates whether the designated column is automatically numbered, thus read-only.boolean
isCaseSensitive
(int column) Indicates whether a column's case matters.boolean
isCurrency
(int column) Indicates whether the designated column is a cash value.boolean
isDefinitelyWritable
(int column) Indicates whether a write on the designated column will definitely succeed.int
isNullable
(int column) Indicates the nullability of values in the designated column.boolean
isReadOnly
(int column) Indicates whether the designated column is definitely not writable.boolean
isSearchable
(int column) Indicates whether the designated column can be used in a where clause.boolean
isSigned
(int column) Indicates whether values in the designated column are signed numbers.boolean
isWrapperFor
(Class<?> iface) boolean
isWritable
(int column) Indicates whether it is possible for a write on the designated column to succeed.toString()
<T> T
-
Method Details
-
getColumnCount
Returns the number of columns in thisResultSet
object.- Specified by:
getColumnCount
in interfaceResultSetMetaData
- Returns:
- the number of columns
- Throws:
SQLException
- if a database access error occurs
-
isAutoIncrement
Indicates whether the designated column is automatically numbered, thus read-only.HSQLDB-Specific Information:
HSQLDB 1.7.1 did not report this value accurately, either always throwing or always returning false, depending upon client property values.
Starting with HSQLDB 1.7.2, this feature is better supported.
However, it must be stated here that, contrary to the generic documentation above, HSQLDB automatically numbered columns (IDENTITY columns, in HSQLDB parlance) are not read-only.In fact, the generic documentation above seems to contradict the general definition of what, at minimum, an auto-increment column is:
Simply, an auto-increment column is one that guarantees it has a unique value after a successful insert or update operation, even if no value is supplied or NULL is explicitly specified by the application or a default value expression.
Further, without SQL Feature T176, Sequence generator support, the attributes of the internal source consulted for unique values are not defined. That is, unlike for a standard SQL SEQUENCE object or a system with full SQL 9x or 200n support for SQL Feature T174, Identity columns, an application must not assume and cannot determine in a standard way that auto-increment values start at any particular point, increment by any particular value or have any of the other attributes generally associated with SQL SEQUENCE generators. Further still, without full support for both feature T174 and T176, if a unique value is supplied by an application or provided by a declared or implicit default value expression, then whether that value is used or substituted with one from the automatic unique value source is implementation-defined and cannot be known in a standard way. Finally, without full support for features T174 and T176, it is also implementation-defined and cannot be know in a standard way whether an exception is thrown or a unique value is automatically substituted when an application or default value expression supplies a non-NULL, non-unique value.
Up to and including HSQLDB 1.7.2, values supplied by an application or default value expression are used if they are indeed non-NULL unique values, while an exception is thrown if either possible value source for the site attempts to supply a non-NULL, non-unique value. This is very likely to remain the behaviour of HSQLDB for its foreseable lifetime and at the very least for the duration of the 1.7.x release series.
Regardless of the new and better support for reporting isAutoIncrement(), it is still possible under certain conditions that accurate reporting may be impossible. For example, if this object's parent Connection is closed before the first call to this method or to any other method of this class that initializes the connection-dependent ResultSetMetaData values, then it is impossible to report accurately for result set columns that directly represent table column values.Under such special circumstances, the driver rethrows the exception that occured during the initialization, or a SQLException wrapping it.
Those wishing to determine the auto-increment status of a table column in isolation from ResultSetMetaData can do so by inspecting the corresponding value of the SYSTEM_COLUMNS.IS_IDENTITY BOOLEAN column which is also currently included (in a fashion proprietary to HSQLDB) as the last column of the jdbcDatabaseMetaData.getColumns() result.
- Specified by:
isAutoIncrement
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
true
if so;false
otherwise- Throws:
SQLException
- if a database access error occurs
-
isCaseSensitive
Indicates whether a column's case matters.HSQLDB-Specific Information:
HSQLDB 1.7.1 did not report this value accurately.
Starting with 1.7.2, this feature is better supported.
This method returns true for any column whose data type is a character type, with the exception of VARCHAR_IGNORECASE for which it returns false. It also returns false for any column whose data type is a not a character data type.
- Specified by:
isCaseSensitive
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
true
if so;false
otherwise- Throws:
SQLException
- if a database access error occurs
-
isSearchable
Indicates whether the designated column can be used in a where clause.HSQLDB-Specific Information:
HSQLDB 1.7.1 did not report this value accurately.
Starting with 1.7.2, this feature is better supported.
If the data type of the column is definitely known to be searchable in any way under HSQLDB, then true is returned, else false. That is, if the type is reported in DatabaseMetaData.getTypeInfo() as having DatabaseMetaData.typePredNone or is not reported, then false is returned, else true.
- Specified by:
isSearchable
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
true
if so;false
otherwise- Throws:
SQLException
- if a database access error occurs
-
isCurrency
Indicates whether the designated column is a cash value.HSQLDB-Specific Information:
Up to and including HSQLDB 1.7.2, this method always returns false.
This is because true fixed (precision,scale) data types are not yet supported. That is, DECIMAL and NUMERIC types are implemented as a thin wrap of java.math.BigDecimal, which cannot, without additional, as yet unimplemented constraint enforcement code, be said to be a fixed (precision,scale) types.
- Specified by:
isCurrency
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
true
if so;false
otherwise- Throws:
SQLException
- if a database access error occurs
-
isNullable
Indicates the nullability of values in the designated column.HSQLDB-Specific Information:
Up to 1.7.1, HSQLDB did not report this value accurately.
Starting with 1.7.2, this feature is better supported.
columnNullableUnknown is always returned for result set columns that do not directly represent table column values (i.e. are calculated), while the corresponding value in SYSTEM_COLUMNS.NULLABLE is returned for result set columns that do directly represent table column values.
Those wishing to determine the nullable status of a table column in isolation from ResultSetMetaData and in a DBMS-independent fashion can do so by calling DatabaseMetaData.getColumns() with the appropriate filter values and inspecting the result at the position described in the API documentation.
- Specified by:
isNullable
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
- the nullability status of the given column; one of
columnNoNulls
,columnNullable
orcolumnNullableUnknown
- Throws:
SQLException
- if a database access error occurs
-
isSigned
Indicates whether values in the designated column are signed numbers.HSQLDB-Specific Information:
HSQLDB 1.7.1 introduced support for this feature and 1.7.2 reports identical values (although using a slightly different implementation).
- Specified by:
isSigned
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
true
if so;false
otherwise- Throws:
SQLException
- if a database access error occurs
-
getColumnDisplaySize
Indicates the designated column's normal maximum width in characters.HSQLDB-Specific Information:
Up to and including HSQLDB 1.7.1, this method always returned 0, which was intended to convey unknown display size. Unfortunately, this value is not universally handled by all clients and in the worst case can cause some applications to crash.
Starting with 1.7.2, this feature is better supported.
The current calculation follows these rules:
- Long character types and datetime types:
The maximum length/precision, repectively.
- CHAR and VARCHAR types:
- If the result set column is a direct pass through of a table
column value, column size was declared and the connection is
to an embedded database instance, then the declared value is
returned.
- Otherwise, the value of the system property
hsqldb.max_xxxchar_display_size or the magic value
32766 (0x7FFE) (tested usable/accepted by most tools and
compatible with assumptions made by java.io read/write
UTF) when the system property is not defined or is not
accessible, due to security constraints.
- If the result set column is a direct pass through of a table
column value, column size was declared and the connection is
to an embedded database instance, then the declared value is
returned.
- Number types:
The max precision, plus the length of the negation character (1), plus (if applicable) the maximum number of characters that may occupy the exponent character sequence. Note that some legacy tools do not correctly handle BIGINT values of greater than 18 digits.
- BOOLEAN (BIT) type:
The length of the character sequence "false" (5), the longer of the two boolean value String representations.
- Remaining types:
The maximum length/precision, respectively, as reported by DatabaseMetaData.getTypeInfo(), when applicable. If the maximum display size is unknown, unknowable or inapplicable, then zero is returned.
- Specified by:
getColumnDisplaySize
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
- the normal maximum number of characters allowed as the width of the designated column
- Throws:
SQLException
- if a database access error occurs
- Long character types and datetime types:
-
getColumnLabel
Gets the designated column's suggested title for use in printouts and displays.HSQLDB-Specific Information:
In HSQLDB a
ResultSet
column label is determined in the following order of precedence:- The label (alias) specified in the generating query.
- The name of the underlying column, if no label is specified.
This also applies to aggregate functions. - An empty
String
.
- Specified by:
getColumnLabel
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
- the suggested column title
- Throws:
SQLException
- if a database access error occurs
-
getColumnName
Get the designated column's name.HSQLDB-Specific Information:
In HSQLDB a ResultSet column name is determined in the following order of prcedence:
- The name of the underlying columnm, if the ResultSet column represents a column in a table.
- The label or alias specified in the generating query.
- An empty
String
.
If the
jdbc.get_column_name
property of the database has been set to false, this method returns the same value asgetColumnLabel(int)
.- Specified by:
getColumnName
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
- column name
- Throws:
SQLException
- if a database access error occurs
-
getSchemaName
Get the designated column's table's schema.HSQLDB-Specific Information:
Up to 1.7.1, HSQLDB did not support the notion of schemas at all, including schema names in result set metadata; this method always returned "".
Staring with 1.7.2, schema name reporting is supported only as an optional, experimental feature that is disabled by default. Enabling this feature requires setting the database property "hsqldb.schemas=true".
Specifically, when this feature is enabled under 1.7.2, only very limited support is provided by the engine for executing SQL containing schema-qualified database object identifiers. That is, when this feature is enabled under 1.7.2, it is not yet possible in most cases to use what would otherwise be the correct, canonical SQL calculated from ResultSetMetaData.
Regardless, reporting is done only in system table content and is not yet carried over to ResultSetMetaData.
For greater detail, see discussion at:
jdbcDatabaseMetaData
.- Specified by:
getSchemaName
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
- schema name or "" if not applicable
- Throws:
SQLException
- if a database access error occurs
-
getPrecision
Get the designated column's number of decimal digits.HSQLDB-Specific Information:
Starting with 1.8.0, HSQLDB reports the the declared length or precision specifiers for table columns (if they are defined, which up to 1.7.2 is not a requirement in DDL), as these values may or may not be enforced, depending on the value of the database property:
sql.enforce_strict_size
Because the property may change from one instantiation of a Database to the next and because, when set true, is not applied to existing values in table columns (only to new values introduced by following inserts and updates), the length and/or precision specifiers for table columns still do not neccessarily accurately reflect true constraints upon the contents of the columns. This situation may or may not change in a future release.- Specified by:
getPrecision
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
- precision
- Throws:
SQLException
- if a database access error occurs
-
getScale
Gets the designated column's number of digits to right of the decimal point.HSQLDB-Specific Information:
Starting with 1.8.0, HSQLDB reports the declared scale for table columns depending on the value of the database property:
sql.enforce_strict_size
- Specified by:
getScale
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
- scale
- Throws:
SQLException
- if a database access error occurs
-
getTableName
Gets the designated column's table name.- Specified by:
getTableName
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
- table name or "" if not applicable
- Throws:
SQLException
- if a database access error occurs
-
getCatalogName
Gets the designated column's table's catalog name.HSQLDB-Specific Information:
Up to and including 1.7.1, HSQLDB did not support the notion of catalog and this method always returned "".
Starting with 1.7.2, HSQLDB supports catalog reporting only as an optional, experimental feature that is disabled by default. Enabling this feature requires setting the database property "hsqldb.catalogs=true". When enabled, the catalog name for table columns is reported as the name by which the hosting Database knows itself.
HSQLDB does not yet support any use of catalog qualification in DLL or DML. This fact is accurately indicated in the corresponding DatabaseMetaData.supportsXXX() method return values.
Regardless, reporting is done only in system table content and is not yet carried over to ResultSetMetaData.
For greater detail, see discussion at:
jdbcDatabaseMetaData
.- Specified by:
getCatalogName
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
- the name of the catalog for the table in which the given column appears or "" if not applicable
- Throws:
SQLException
- if a database access error occurs
-
getColumnType
Retrieves the designated column's SQL type.HSQLDB-Specific Information:
This reports the SQL type of the column. HSQLDB can return Objects in any Java integral type wider than
Integer
for an SQL integral type.- Specified by:
getColumnType
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
- SQL type from java.sql.Types
- Throws:
SQLException
- if a database access error occurs- See Also:
-
getColumnTypeName
Retrieves the designated column's database-specific type name.- Specified by:
getColumnTypeName
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
- type name used by the database. If the column type is a user-defined type, then a fully-qualified type name is returned.
- Throws:
SQLException
- if a database access error occurs
-
isReadOnly
Indicates whether the designated column is definitely not writable.HSQLDB-Specific Information:
Up to and including 1.7.1, HSQLDB did not report this value accurately.
Starting with HSQLDB 1.7.2, this feature is better supported.
For result set columns that do not directly represent table column values (i.e. are calculated), true is reported. Otherwise, the read only status of the table and the database are used in the calculation, but not the read-only status of the session.
- Specified by:
isReadOnly
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
true
if so;false
otherwise- Throws:
SQLException
- if a database access error occurs
-
isWritable
Indicates whether it is possible for a write on the designated column to succeed.HSQLDB-Specific Information:
Up to and including 1.7.1, HSQLDB did not report this value accurately.
Starting with HSQLDB 1.7.2, this feature is better supported.
In essense, the negation of isReadOnly() is reported.
- Specified by:
isWritable
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
true
if so;false
otherwise- Throws:
SQLException
- if a database access error occurs
-
isDefinitelyWritable
Indicates whether a write on the designated column will definitely succeed.HSQLDB-Specific Information:
HSQLDB 1.7.1 did not report this value accurately.
Starting with HSQLDB 1.7.2, this method always returns false. The reason for this is that it is generally either very complex or simply impossible to calculate deterministically true for table columns under all concievable conditions. The value is of dubious usefulness, except perhaps if there were support for updateable result sets using "SELECT ... FOR UPDATE" style locking. However, this is not anticipated to occur any time in the 1.7.x release series.
- Specified by:
isDefinitelyWritable
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
true
if so;false
otherwise- Throws:
SQLException
- if a database access error occurs
-
getColumnClassName
Returns the fully-qualified name of the Java class whose instances are manufactured if the methodResultSet.getObject
is called to retrieve a value from the column.ResultSet.getObject
may return a subclass of the class returned by this method.HSQLDB-Specific Information:
HSQLDB 1.7.1 did not support this feature; calling this method always caused an
SQLException
to be thrown, stating that the function was not supported.- Specified by:
getColumnClassName
in interfaceResultSetMetaData
- Parameters:
column
- the first column is 1, the second is 2, ...- Returns:
- the fully-qualified name of the class in the Java programming
language that would be used by the method
ResultSet.getObject
to retrieve the value in the specified column. This is the class name used for custom mapping. - Throws:
SQLException
- if a database access error occurs- Since:
- JDK 1.2 (JDK 1.1.x developers: read the new overview for jdbcResultSet)
-
toString
-
unwrap
- Specified by:
unwrap
in interfaceWrapper
- Throws:
SQLException
-
isWrapperFor
- Specified by:
isWrapperFor
in interfaceWrapper
- Throws:
SQLException
-