More specifically, what data structure does MYSQL use for multi-column indexing? I know MYSQL use Btree for indexing, which can only index one column. How does multi-column indexing work then? Answer Think of a MySQL “composite” index this way. Concatenate all the columns in the index together, th…
Tag: mysql
MySQL JOINED table query should return zero but instead does not return those rows
I have two tables, saved_groups and user_comments and the second line of the query should (does, it works) return an extra column with the number of comments associated with each group. However, when a group has zero comments associated with it, that group is simply not returned. The only rows returned are th…
Check value of last 4 rows
I want to check the value of the last 4 rows from DB. If these 4 rows have a specific value, Do something. So the table looks like this: Here is a fiddle to test with: http://sqlfiddle.com/#!9/1064b3/1 I can run the following query SELECT * FROM testing ORDER BY id DESC LIMIT 4, Then check with PHP: Is there …
SQL Error 3780 when using foreign key as primary key
I’m getting the following order trying to use a table’s primary key as another table’s primary key: ERROR 3780 (HY000): Referencing column ‘optimization_id’ and referenced column ‘optimization_id’ in foreign key constraint ‘optimization.main – optimization…
Error(1064) (42000) : check the manual that corresponds to your MySQL server version for the right syntax to use near ‘)’ at line 9
My sql table is giving error but I cant find one ! This my sql table : Error: mysql> SOURCE /home/q55555b/Documents/Database Systems/Final Assesment_20193113/createawardprize.sql ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the…
Delete all SQL rows except one for a Group
I have a table like this: Schema (MySQL v5.7) id userid date 1 1 2021-11-15 2 2 2021-11-15 3 1 2021-11-13 4 3 2021-10-13 5 3 2021-09-13 6 2 2021-09-13 View on DB Fiddle I want to delete all records which are older than 14 days, EXCEPT if the user only has records which are older – than keep the
Mysql Select X rows after specific match
I’m trying to write a select statement in MySQL to get 5 rows after I get my match then sum how many times those numbers were repeated. Example: Raw Table id number 1 1 2 0 3 9 4 14 5 11 6 0 7 3 8 4 9 10 10 9 11 0 12 5 13 3 14 11
Slow Querying DB
I am currently optimising a system with many connected tables. The part that I am working on right now is displaying table orders. The problem is that in this table there are also many relations (around 10) which I am querying. The problem itself is in querying that many relations. I have been using Eloquent …
How to find the Employee names and their supervisor names if the table doesn’t have common numeric column like Employee_id or employee number in Mysql
If the table have only two columns with employee name and their supervisor column and if it doesn’t have any other numeric or number column with employee_number or employee_id, then how the results can be produced. I’m not getting logic to show the results. Code for creating table in Mysql: ItR…
MySQL match a value with wildcard in multiple columns
I have a teachers table that looks like this: teacherid teacherfname teacherlname salary 1 Alexander Bennett 55.30 I would like to return any record that contains a given string in the teacherfname, teacherlname and salary columns. What I have right now (this returns exact match only): What I would like to do…