rdbms

问题 I have a database with the following structure ID Name 1 John 2 Doe 3 Dave 4 Smith Another table holds employee relatives ID EmpID RelativeType Name 1 1 Son x 2 1 Daughter y 3 1 Wife a 4 1 Friend b 5 1 Father c 6 1 Friend e Childs can be fixed upto maximum of 8 and friends can be maximum of 3 I was able to make a pivot query below to bring childs, but unable to pivot again to fetch 3 friends, father etc. This is my query: select * from ( select e.ID, e.FirstName, CASE WHEN es.RelativeType =

问题 Is it right to expect that, for the typical modern RDBMS, querying by one specific primary key is as fast as querying a hashtable by key? Or is there "actual work" done to traverse the table and track down the primary key value? That seems unthinkably wasteful, even if there are automatic indexes for primary keys. 回答1: Database operation involves access of secondary memory unit (Disk). And to achieve efficiency important is to reduce block access time(not operations). Complexity of Select

问题 Is it right to expect that, for the typical modern RDBMS, querying by one specific primary key is as fast as querying a hashtable by key? Or is there "actual work" done to traverse the table and track down the primary key value? That seems unthinkably wasteful, even if there are automatic indexes for primary keys. 回答1: Database operation involves access of secondary memory unit (Disk). And to achieve efficiency important is to reduce block access time(not operations). Complexity of Select

问题 User Table: ID Name 1 usr1 2 usr2 3 usr3 In the above table, ID is a primary key. My requirement is while inserting data into the table, I would like to specify only the name like INSERT INTO user VALUES('usr4') . After execute the query, Is there any way to automatically create ID for 'usr4'? I tried with serial data type. For that also we have to specify the keyword default like INSERT INTO user VALUES(default,'usr4') . So, is there any way to do like INSERT INTO user VALUES('usr4') . NOTE:

问题 I have a GIN index set up for full text search. I would like to get a list of records that match a search query, ordered by rank (how well the record matched the search query). For the result, I only need the record and its columns, I do not need the actual rank value that was used for ordering. I have the following query, which runs fine and returns the expected results from my postgresql db. SELECT *, ts_rank('{0.1,0.1,0.1,0.1}', users.textsearchable_index_col, to_tsquery('smit:* | ji:*'))

问题 I've created a temporary table DETAILS and follow the same syntax of creating and inserting in it. But I have not received any result set However, the CREATE and INSERT statements ran successfully and the Row was also affected in the INSERT statement . But the result set was empty when I ran the last SELECT statement to view the record . DROP TABLE DETAILS ; CREATE GLOBAL TEMPORARY TABLE DETAILS AS ( SELECT ins_id , firstname , pages FROM INSTRUCTOR)DEFINITION ONLY; INSERT INTO DETAILS SELECT

问题 I've created a temporary table DETAILS and follow the same syntax of creating and inserting in it. But I have not received any result set However, the CREATE and INSERT statements ran successfully and the Row was also affected in the INSERT statement . But the result set was empty when I ran the last SELECT statement to view the record . DROP TABLE DETAILS ; CREATE GLOBAL TEMPORARY TABLE DETAILS AS ( SELECT ins_id , firstname , pages FROM INSTRUCTOR)DEFINITION ONLY; INSERT INTO DETAILS SELECT

问题 I am trying to run a foreign key add query as below, with foreign key checks set to 0. Both the columns in the two tables are exactly the same. Also, both are primary keys. None of the solutions here helped in solving this problem. I'm on localhost. mysql> alter table deliveryaddress -> add foreign key(oid) references productorder(oid) -> on delete cascade on update restrict; ERROR 1822 (HY000): Failed to add the foreign key constaint. Missing index for c onstraint '' in the referenced table

问题 I am trying to run a foreign key add query as below, with foreign key checks set to 0. Both the columns in the two tables are exactly the same. Also, both are primary keys. None of the solutions here helped in solving this problem. I'm on localhost. mysql> alter table deliveryaddress -> add foreign key(oid) references productorder(oid) -> on delete cascade on update restrict; ERROR 1822 (HY000): Failed to add the foreign key constaint. Missing index for c onstraint '' in the referenced table

来源： https://stackoverflow.com/questions/6139597/should-varchar-columns-be-put-at-the-end-of-table-definitions-in-mysql