Sql updating joined tables
Sometimes you have a lot of processes that could be updating a column value in a table. ID=54321 Here's a workaround for the update/subquery/cant do self table "bug"Senario is, ID 8 has multiple records, only the last (highest) record needs to be changedupdate t1 set c1 = ' NO'where id='8'order by recno desc limit 1I would prefer update t1 set c1=' NO' WHERE ID=8 AND RECNO = (SELECT MAX(RECNO) FROM T1 WHERE ID=8)But that's not currently allowed If you want to update a table based on an aggregate function applied to another table, you can use a correlated subquery, for example: UPDATE table1 SET table1field = (SELECT MAX(table2.table2field) FROM table2 WHERE table1.table1field = table2.table2field)This can be helpful if you need to create a temporary table storing an ID (for, say, a person) and a "last date" and already have another table storing all dates (for example, all dates of that person's orders).
If you want to return the value before you updated it without using a seperate select (which unless you lock the table could return a different value than is updated) then you can use a mysql variable like this:update some_table set col = col 1 where key = 'some_key_value' and @value := col The @value := col will always evaluate to true and will store the col value before the update in the @value variable. Additional information on My SQL correlated subqueries is at UPDATE can apparently be used to implement a semaphore (pardon my pseudocode):while TRUE The code above waits until the semaphore is "cleared" (value = 0) and then "sets" it (value = 1).
You can use SQL JOIN statements to combine data from three or more tables.
SQL JOIN is extremely flexible, and its powerful functionality can be used to combine data from multiple tables.
The inner join occurs where both the vehicle and driver are located in the same city.
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(This is because the order in which the rows are updated determines which rows are ignored.) Such statements produce a warning in the error log when using statement-based mode and are written to the binary log using the row-based format when using mode.
(Bug #11758262, Bug #50439) See Section 18.104.22.168, “Determination of Safe and Unsafe Statements in Binary Logging”, for more information.
You could bring a third table into your query by extending the JOIN statement as follows: SELECT lastname, firstname, tag, open_weekends FROM drivers, vehicles, locations WHERE drivers.location = vehicles.location AND vehicles.location = locations.location AND locations.open_weekends = ' Yes' This powerful extension to the basic SQL JOIN statement allows you to combine data in a complex manner.
In addition to combining tables with an inner join, you can also use this technique to combine multiple tables using an outer join.