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Even if you think you know everything about LEFT JOIN, I bet you will learn something or two in this post! A reminder about “A LEFT JOIN B ON conditional_expr” The ON condition (in the expression “A LEFT JOIN B ON conditional_expr”) is used to decide how to retrieve rows from table B (Matching-Stage). If [...]
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Query Optimizer is a part of the server that takes a parsed SQL query and produces a query execution plan. MySQL Query Optimizer uses (as one of its parameters) the stored key distribution (Cardinality) mechanism to determine the order in which tables should be joined, when you perform a join on something other than a [...]
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The MySQL IN() Comparison Operator is said to be very quick if all the values are constants (the values are then evaluated and sorted first, and the search is done using a binary search). However, what if the field which the IN clause refers to, is part of the index used to execute the query? [...]
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A SELECT statement with COUNT returns the number of rows retrieved by the SELECT statement. For performance, the desired result is to limit that count. Including a LIMIT clause in the SELECT statement will not work since it only restricts the number of rows returned, which is always one. The solution, what I call “Limited-Count”, is done [...]
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One way to do a “Conditional Join” in MySQL is by using a “LEFT JOIN”.  Create a “LEFT JOIN” for each condition and combine the results into one column using an “IF” statement by the “SELECT” expression.  Here’s an example: Suppose you have three tables: questions: a table consisting of question ids, timestamps, and whether or not [...]
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  Question: How can you select records from one table that doesn’t have a specific ID in a second, many-to-many table?   In other words, imagine you have a questions table (A), categories table (B), and a many-to-many relationship link table (L).  This setup allows a question to have several categories.  How can you find questions that don’t [...]
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