Multi-Layer Nested Subquery

Function

This statement is used to nest queries in the subquery.

Syntax

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SELECT attr_expr FROM ( SELECT attr_expr FROM ( SELECT attr_expr FROM... ... ) [alias] ) [alias];

Keyword

• All is used to return repeated rows. By default, all repeated rows are returned. It is followed by asterisks (*) only. Otherwise, an error will occur.
• DISTINCT is used to remove the repeated line from the result.

Precautions

• The to-be-queried table must exist. If this statement is used to query a table that does not exist, an error is reported.
• The alias of the subquery must be specified in the nested query. Otherwise, an error is reported.
• The alias must be specified before the running of the statement. Otherwise, an error is reported. It is advised to specify a unique alias.

Example

To return the name field from the user_info table after three queries, run the following statement:

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SELECT name FROM ( SELECT name, acc_num FROM ( SELECT name, acc_num, password FROM ( SELECT name, acc_num, password, bank_acc FROM user_info) a ) b ) c;