select from two tables sql without join

Today I want to check all the Realname Username,psw,gname,tel Table structure: Table T1 Field Name: T1_ID,USERNAME,PSW Table T2 Field Name: t2_id,gname,t1_id//Here a t1_id corresponds to multiple t2_id T3 Field Name: t3_id,realname,tel,t1_id//Here a t1_id corresponds to a t3_id The trouble is when a realname in the T2 table does not have content, but also to the Username,psw,tel of this realname display, direct use Sql= "

Column Based on the SELECT column to return the final result. Second,Two-table join query: the product (Cartesian Product) of the two tables is filtered using the ON condition and connection type to form an intermediate table. Then, the records of the intermediate table are filtered Based ON the WHERE condition, return the query result based on the column specif

= ' Your table name ' To determine the existence of a field: Select COUNT (*) from syscolumns where ID = (SELECT id from sysobjects where type= ' u ' and name= ' your table name ') and name = ' field name you want to judge ' A small example --Suppose the table name you want to work with is: Tb --Determine if there is a primary key in the table to add the column if exists (

); then select the corresponding Column Based on the Select column to return the final result.2. join queries for two tables: product (Cartesian Product) of the two tables and filter using the on condition and connection type to form an intermediate table. Then, filter reco

condition to form an intermediate table (this intermediate table is invisible to users ); then SELECT the corresponding column based on the SELECT column to return the final result.Second,Two-table join query: the product (Cartesian product) of the two tables is filtered using the ON condition and connection type to f

The syntax is as follows: 1 Description: copy a table (only copy structure, source table name: a new table name: B) (access available)Method 1: Select * Into B From A Where 1 1 Method 2: Select Top 0 * Into B From A 2Description: Copy table (copy data, source table name: A target table name: B) (access available)Insert IntoB (A, B, C)SelectD, E, FFromA; 3.

right table, regardless of whether the table on the left has matching data: Select S.name,m.mark from student s right join Mark M on S.id=m.studentid Iv. fully connected-full join: Using the format as above has been explained above Remove the data from the left and right two tables, whether or not they match:

two results were met, and the results were as follows: Second, left join connect-left join: Left join is to select all the tuples from the left table: Select S.name,m.mark from student s left join Mark M on S.id=m.studentid The

. Next, look at the Orders table: Id_o OrderNo id_p 1 77895 3 2 44678 3 3 22456 1 4 24562 1 5 34764 65 Note that the "id_o" column is the primary key in the Orders table, and the "id_p" column in the Orders table is used to refer to people in the "Persons" table without using their exact names. Please note that the "id_p" column in th

-right join: The right connection is to remove all data from the right table, regardless of whether the table on the left has matching data: Select S.name,m.mark from student s right join Mark M on S.id=m.studentid The above statement takes all the data from the Mark score table, regardless of whether there is a data match in the student table, as shown in: Iv.

: The right connection is to remove all data from the right table, regardless of whether the table on the left has matching data: Select S.name,m.mark from student s right join Mark M on S.id=m.studentid The above statement takes all the data from the Mark score table, regardless of whether there is a data match in the student table, as shown in: Iv. fully connected-full

-conforming values are displayed as null.4. Complete External join: full join or full outer join (1) The Complete External join returns all rows in the left and right tables. If a row does not match a row in another table, the selection list column of the other table contain

Left JOIN, Inner join related content, very practical, for understanding the principle and specific applications are very helpful! The LEFT join is a shorthand for the left outer join, and the left join defaults to the outer property. The Inner

a20050115 NULL NULL(The number of rows affected is 5 rows)Result Description:The left join is based on the records of Table A, a can be regarded as the right table, and B can be regarded as left table.In other words, the records of the left table (A) will all be represented, and the right table (B) will only display records that match the search criteria (in the example: A.aid = b.bid).The low-record of table B is null.2. Right JoinThe