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This article mainly introduces the MySQL database SELECT query expression example analysis, has a certain reference value, interested friends can refer to, I hope you can learn a lot after reading this article, the following let the editor with you to understand.

A large part of data management is searching, and SELECT accounts for a large part of it.

SELECT select_expr [, select_expr...] [FROM table_reference WHERE [where_condition] [GROUP BY {col_name | position} [ASC | DESC],...] [HAVING where_condition] [ORDER BY {col_name | expr | position} [ASC | DESC],...] [LIMIT {[offset,] row_count | row_count OFFSET offset}]]

So how do select_expr query expressions be written?

a. Each expression represents a desired column, and there must be at least one

b. Multiple columns are separated by English commas

In the user data table, execute to find only the first two columns

Root@localhost test > select id,username from user

Of course, the order of query expressions can be inconsistent with the order in the data table, then the query results are displayed according to the results of query expressions, that is, the order of SELECT query expressions will affect the order of query results.

Root@localhost test > select username,id from user

c. An asterisk (*) represents all columns, and table_name.* can represent all columns of a named table

Root@localhost test > SELECT * FROM user; root@localhost test > SELECT user.id,user.username FROM user

Since the field name has been specified here, why specify the name of the data table user in user.id and use.name? Because if there are multiple table joins, that is, two different tables are stored in the same field. If you write the field name directly, you may not know which data table the field belongs to, so you can tell which data table the field belongs to by adding the data table name.

d. Query expressions can be aliased using [AS] alias_name

Root@localhost test > SELECT id AS userID,username AS Uname FROM user

It is found that the original id,username in the table has become userID,Uname, so the alias also affects the result.

Please pay attention to the grammar of aliases here.

Root@localhost test > SELECT id username FROM user

At this point, username appears as an alias for id, which means that if the alias coincides with a field that does exist in the data table, username now refers to the alias rather than the real field.

e. Aliases can be used in GROUP BY, ORDER BY, or HAVING clauses

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