Problems with select statements in SQL?

Generally refers to indexing. You don't have to.

Query all fields and data in the table following from;

select * from test1, all fields and data in the test1 table will be displayed.

The basic format is: select field name from table name.

For example, select first name from employees.

select * from employees。

select first evaluates the output expression for each selected row.

Using union, intersect, and minus, we can combine the output of multiple select statements into a single result set. The union operator returns rows in two or one of the result sets, and the intersect operator returns rows that are strictly present in both result sets. The minus operator returns rows that are in the first result set, but not in the second result set.

In either case, duplicate lines are removed unless all is declared.

Query all the data in the table following from.

Select is an instruction used in SQL Data Manipulation Language (DML) to query field data in **, which can be used with conditional clauses (such as where) or sequential clauses (such as order) to obtain query results.

The basic format of the select statement is as follows:

select [all |distinct] column name [, column name. from Data Table Name [, Data Table Name. where filter condition] [group by column name[, column name.

order by FieldName[,FieldName.。

Introduction to the functions of the select statement

Column selection (projection operation): Ability to use the column selection function of the select statement to select the columns in the table that we want to return with the query. When we query, we can select the columns specified in the table for the query.

Row selection (select operation): Ability to use the row selection function of the select statement to select rows in the table that we want to return with the query. Ability to use different criteria to limit the rows seen.

Join (multi-table operation): You can use the join function of the select statement to aggregate data, which is stored in different tables, but we can query the data through the join.

For the above content, please refer to Encyclopedia—select().