Discussion of left and right connection and inner connection

The left join connection and the right connection have been two tables, a and a left link: a left joins B on A.aid=b.bid; the Zuo join statement can be viewed as a table, using the from call, or as a bracelet multiple times using the left JOIN, if the field B table in table A does not correspond, Then use NULL to fill the left and right connections can be used to explain the model, taking left join as an example: a LEFT JOIN B on A.aid=b.bid;a table on the right side, B table on the other hand according to the conditions of the data in table a match, get the final result if the field is taken out in a, B, You need to indicate which table is in the field, or else you will get an error. Left and right connections are relative, a ieft join B on ...; Equal to B right a on ...; The left connection is usually used because of the compatibility and the general way of thinking within the person connected: a inner B on ...; The inner connection can be regarded as the intersection of a, b, or so, that is, a B matches each other, that is, the null value of both

Discussion of left and right connection and inner connection