![]() SELECTPurpose. Use a SELECT statement or subquery to retrieve data from one or more tables, object tables, views, object views, or materialized views. If part or all of the result of a SELECT statement is equivalent to an existing materialized view, then Oracle Database may use the materialized view in place of one or more tables specified in the SELECT statement. This substitution is called query rewrite. It takes place only if cost optimization is enabled and the QUERY. To determine whether query write has occurred, use the EXPLAINPLAN statement. Prerequisites. For you to select data from a table or materialized view, the table or materialized view must be in your own schema or you must have the SELECT privilege on the table or materialized view. For you to select rows from the base tables of a view: You must have the SELECT privilege on the view, and. Whoever owns the schema containing the view must have the SELECT privilege on the base tables. The SELECTANYTABLE system privilege also allows you to select data from any table or any materialized view or the base table of any view. To issue an Oracle Flashback Query using the flashback. In addition, either you must have FLASHBACK object privilege on the objects in the select list, or you must have FLASHBACKANYTABLE system privilege. Syntaxselect: :=Description of the illustration select. You can then reference the subquery block multiple places in the query by specifying the query name. Oracle Database optimizes the query by treating the query name as either an inline view or as a temporary table. You can specify this clause in any top- level SELECT statement and in most types of subqueries. The query name is visible to the main query and to all subsequent subqueries except the subquery that defines the query name itself. Restrictions on Subquery Factoring This clause is subject to the following restrictions: You can specify only one subquery. You cannot specify a query. How to UPDATE using ROWNUM and ORDER BY. Pro*C does not process the 'ORDER BY' statement. ![]() These two keywords are synonymous. Duplicate rows are those with matching values for each expression in the select list. Restrictions on DISTINCT and UNIQUE Queries These types of queries are subject to the following restrictions: When you specify DISTINCT or UNIQUE, the total number of bytes in all select list expressions is limited to the size of a data block minus some overhead. This size is specified by the initialization parameter DB. The default is ALL.* (all- column wildcard)Specify the all- column wildcard (asterisk) to select all columns, excluding pseudocolumns, from all tables, views, or materialized views listed in the FROM clause. The columns are returned in the order indicated by the COLUMN. You must have specified the subquery. Oracle Database returns a set of columns in the order in which the columns were specified when the object was created. A query that selects rows from two or more tables, views, or materialized views is a join. You can use the schema qualifier to select from a table, view, or materialized view in a schema other than your own. If you omit schema, then the database assumes the table, view, or materialized view is in your own schema. Specify an expression representing the information you want to select. A column name in this list can be qualified with schema only if the table, view, or materialized view containing the column is qualified with schema in the FROM clause. If you specify a member method of an object type, then you must follow the method name with parentheses even if the method takes no arguments. Oracle Database will use this alias in the column heading of the result set. The AS keyword is optional. The alias effectively renames the select list item for the duration of the query. Bulk Processing with BULK COLLECT and. I will show you how you can use PL/SQL’s bulk processing. Use a FORALL statement to update all the rows identified.The alias can be used in the order. Any columns in the select list of the subquery that are not needed by the GROUPBY operation are ignored without error. Expressions involving the preceding expressions that evaluate to the same value for all rows in a group. You can select a rowid from a join view only if the join has one and only one key- preserved table. The rowid of that table becomes the rowid of the view. If two or more tables have some column names in common, and if you are specifying a join in the FROM clause, then you must qualify column names with names of tables or table aliases. FROM Clause. The FROM clause lets you specify the objects from which data is selected. Specify ONLY if the view in the FROM clause is a view belonging to a hierarchy and you do not want to include rows from any of its subviews. You can also implement session- level Flashback using the DBMS. You can retrieve the corresponding identifier of the transaction that made the change using the VERSIONS. You can also retrieve information about the transaction that resulted in a particular row version by issuing an Oracle Flashback Transaction Query. ![]() You do this by querying the FLASHBACK. If you specify SCN, then expr must evaluate to a number. If you specify TIMESTAMP, then expr must evaluate to a timestamp value. Oracle Database returns rows as they existed at the specified system change number or time. VERSIONS Specify VERSIONS to retrieve multiple versions of the rows returned by the query. Oracle Database returns all committed versions of the rows that existed between two SCNs or between two timestamp values. The rows returned include deleted and subsequently reinserted versions of the rows. Specify BETWEENSCN .. Both expressions must evaluate to a number. MINVALUE and MAXVALUE resolve to the SCN of the oldest and most recent data available, respectively. Specify BETWEENTIMESTAMP .. Both expressions must evaluate to a timestamp value. ![]() MINVALUE and MAXVALUE resolve to the timestamp of the oldest and most recent data available, respectively. Oracle Database provides a group of version query pseudocolumns that let you retrieve additional information about the various row versions. The VERSIONS clause determines the versions of the rows as seen from the ASOF point. The database returns null for a row version if the transaction started before the first BETWEEN value or ended after the ASOF point. Restrictions on Flashback Queries These queries are subject to the following restrictions: You cannot specify a subquery in the expression of the ASOF clause. You cannot use the VERSIONS clause in flashback queries to temporary or external tables, or tables that are part of a cluster.
You cannot use the VERSIONS clause in flashback queries to views. However, you can use the VERSIONS syntax in the defining query of a view. You cannot specify this clause if you have specified query. Oracle Database will interpret the condition and fetch data from only those partitions. It is not possible to formulate such a WHERE condition for hash- partitioned data. For dblink, specify the complete or partial name for a database link to a remote database where the table, view, or materialized view is located. This database need not be an Oracle Database. If you omit dblink, then the database assumes that the table, view, or materialized view is on the local database. Restrictions on Database Links Database links are subject to the following restrictions: You cannot query a user- defined type or an object REF on a remote table. You cannot query columns of type ANYTYPE, ANYDATA, or ANYDATASET from remote tables. If a more efficient execution path exists, then Oracle Database does not perform block sampling. If you want to guarantee block sampling for a particular table or index, then use the FULL or INDEX. The value must be in the range . This percentage indicates the probability of each row, or each cluster of rows in the case of block sampling, being selected as part of the sample. It does not mean that the database will retrieve exactly sample. If you omit this clause, then the resulting sample will change from one execution to the next. Restriction on Sampling During Queries When sampling from a view, you must ensure that the view is key preserved. One way to do this is to use a CREATE TABLE .. ASsubquery statement to materialize the result of an arbitrary query and then perform sampling on the resulting query. Restrictions on sample. When used in the subquery of a DML statement, you can specify this clause in a subquery in the FROM clause but not in subquery in the WHERE clause. CONSTRAINT constraint Specify the name of the CHECK OPTION constraint. If you omit this identifier, then Oracle automatically assigns the constraint a name of the form SYS. Regardless of its form, it must return a collection value—that is, a value whose type is nested table or varray. This process of extracting the elements of a collection is called collection unnesting. The optional plus (+) is relevant if you are joining the TABLE expression with the parent table. The + creates an outer join of the two, so that the query returns rows from the outer table even if the collection expression is null. Note. In earlier releases of Oracle, when collection. That usage is now deprecated. The collection. This is called left correlation. Left correlation can occur only in table. Other subqueries cannot contains references to columns defined outside the subquery. The optional (+) lets you specify that table. The (+) is valid only if collection. The result is similar to that of an outer join. When you use the (+) syntax in the WHERE clause of a subquery in an UPDATE or DELETE operation, you must specify two tables in the FROM clause of the subquery. Oracle Database ignores the outer join syntax unless there is a join in the subquery itself. This alias is required if the select list references any object type attributes or object type methods. Correlation names are most often used in a correlated query. Other references to the table, view, or materialized view throughout the query must refer to this alias. The output of a pivot operation typically includes more columns and fewer rows than the starting data set. Aggregation functions must specify a GROUPBY clause to return multiple values, yet the pivot.
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