Su, Xiaomeng 04d4597cf3 dli_sqlreference_0511_version

Reviewed-by: Pruthi, Vineet <vineet.pruthi@t-systems.com>
Co-authored-by: Su, Xiaomeng <suxiaomeng1@huawei.com>
Co-committed-by: Su, Xiaomeng <suxiaomeng1@huawei.com>

2023-11-02 14:34:08 +00:00

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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;

Parent topic: Subquery

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