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9.2.1 Identifier Qualifiers

Inhoudsopgave

Object names may be unqualified or qualified. An unqualified name is permitted in contexts where interpretation of the name is unambiguous. A qualified name includes at least one qualifier to clarify the interpretive context by overriding a default context or providing missing context.

For example, this statement creates a table using the unqualified name t1:

  1. CREATE TABLE t1 (i INT);

Because t1 includes no qualifier to specify a database, the statement creates the table in the default database. If there is no default database, an error occurs.

This statement creates a table using the qualified name db1.t1:

  1. CREATE TABLE db1.t1 (i INT);

Because db1.t1 includes a database qualifier db1, the statement creates t1 in the database named db1, regardless of the default database. The qualifier must be specified if there is no default database. The qualifier may be specified if there is a default database, to specify a database different from the default, or to make the database explicit if the default is the same as the one specified.

Qualifiers have these characteristics:

  • An unqualified name consists of a single identifier. A qualified name consists of multiple identifiers.

  • The components of a multiple-part name must be separated by period (.) characters. The initial parts of a multiple-part name act as qualifiers that affect the context within which to interpret the final identifier.

  • The qualifier character is a separate token and need not be contiguous with the associated identifiers. For example, tbl_name.col_name and tbl_name . col_name are equivalent.

  • If any components of a multiple-part name require quoting, quote them individually rather than quoting the name as a whole. For example, write `my-table`.`my-column`, not `my-table.my-column`.

  • A reserved word that follows a period in a qualified name must be an identifier, so in that context it need not be quoted.

The permitted qualifiers for object names depend on the object type:

  • A database name is fully qualified and takes no qualifier:

    1. CREATE DATABASE db1;

  • A table, view, or stored program name may be given a database-name qualifier. Examples of unqualified and qualified names in CREATE statements:

    1. CREATE TABLE mytable ...;
    2. CREATE VIEW myview ...;
    3. CREATE PROCEDURE myproc ...;
    4. CREATE FUNCTION myfunc ...;
    5. CREATE EVENT myevent ...;
    6.  
    7. CREATE TABLE mydb.mytable ...;
    8. CREATE VIEW mydb.myview ...;
    9. CREATE PROCEDURE mydb.myproc ...;
    10. CREATE FUNCTION mydb.myfunc ...;
    11. CREATE EVENT mydb.myevent ...;

  • A trigger is associated with a table, so any qualifier applies to the table name:

    1. CREATE TRIGGER mytrigger ... ON mytable ...;
    2.  
    3. CREATE TRIGGER mytrigger ... ON mydb.mytable ...;

  • A column name may be given multiple qualifiers to indicate context in statements that reference it, as shown in the following table.

    Column Reference Meaning
    col_name Column col_name from whichever table used in the statement contains a column of that name
    tbl_name.col_name Column col_name from table tbl_name of the default database
    db_name.tbl_name.col_name Column col_name from table tbl_name of the database db_name

    In other words, a column name may be given a table-name qualifier, which itself may be given a database-name qualifier. Examples of unqualified and qualified column references in SELECT statements:

    1. SELECT c1 FROM mytable
    2. WHERE c2 > 100;
    3.  
    4. SELECT mytable.c1 FROM mytable
    5. WHERE mytable.c2 > 100;
    6.  
    7. SELECT mydb.mytable.c1 FROM mydb.mytable
    8. WHERE mydb.mytable.c2 > 100;

You need not specify a qualifier for an object reference in a statement unless the unqualified reference is ambiguous. Suppose that column c1 occurs only in table t1, c2 only in t2, and c in both t1 and t2. Any unqualified reference to c is ambiguous in a statement that refers to both tables and must be qualified as t1.c or t2.c to indicate which table you mean:

  1. SELECT c1, c2, t1.c FROM t1 INNER JOIN t2
  2. WHERE t2.c > 100;

Similarly, to retrieve from a table t in database db1 and from a table t in database db2 in the same statement, you must qualify the table references: For references to columns in those tables, qualifiers are required only for column names that appear in both tables. Suppose that column c1 occurs only in table db1.t, c2 only in db2.t, and c in both db1.t and db2.t. In this case, c is ambiguous and must be qualified but c1 and c2 need not be:

  1. SELECT c1, c2, db1.t.c FROM db1.t INNER JOIN db2.t
  2. WHERE db2.t.c > 100;

Table aliases enable qualified column references to be written more simply:

  1. SELECT c1, c2, t1.c FROM db1.t AS t1 INNER JOIN db2.t AS t2
  2. WHERE t2.c > 100;

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Document heeft de 26/06/2006 gemaakt, de laatste keer de 26/10/2018 gewijzigd
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  1. Bekijk - html-document Taal van het document:en Manuel MySQL : https://dev.mysql.com/

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