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Can a foreign key be NULL and/or duplicate?
...mply requires that the value in that field must exist first in a different table (the parent table). That is all an FK is by definition. Null by definition is not a value. Null means that we do not yet know what the value is.
Let me give you a real life example. Suppose you have a database that sto...
How to align content of a div to the bottom
...ignment issues with the items aren't fixed height, it's easier just to use tables.
Example: Can you do this HTML layout without using tables?
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Why is SELECT * considered harmful?
...work. This is especially true when someone adds new columns to underlying tables that didn't exist and weren't needed when the original consumers coded their data access.
Indexing issues. Consider a scenario where you want to tune a query to a high level of performance. If you were to use *, and ...
Count table rows
What is the MySQL command to retrieve the count of records in a table?
11 Answers
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Html table tr inside td
I am trying to create a table in HTML. I have the following design to create. I had added a <tr> inside the <td> but somehow the table is not created as per the design.
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How to get all columns' names for all the tables in MySQL?
Is there a fast way of getting all column names from all tables in MySQL , without having to list all the tables?
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Effect of NOLOCK hint in SELECT statements
...2) Yes, a select with NOLOCK will allow other queries against the effected table to complete faster than a normal select.
Why would this be?
NOLOCK typically (depending on your DB engine) means give me your data, and I don't care what state it is in, and don't bother holding it still while you re...
How many database indexes is too many?
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It depends on the operations that occur on the table.
If there's lots of SELECTs and very few changes, index all you like.... these will (potentially) speed the SELECT statements up.
If the table is heavily hit by UPDATEs, INSERTs + DELETEs ... these will be very slow w...
Multiple Updates in MySQL
... use INSERT ... ON DUPLICATE KEY UPDATE.
Using your example:
INSERT INTO table (id,Col1,Col2) VALUES (1,1,1),(2,2,3),(3,9,3),(4,10,12)
ON DUPLICATE KEY UPDATE Col1=VALUES(Col1),Col2=VALUES(Col2);
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How do I show the schema of a table in a MySQL database?
From the MySQL console, what command displays the schema of any given table?
5 Answers
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