Advanced queries in MySQL

Advanced queries in MySQL

Previously learned query syntax: Select field name from table name where condition

In fact, the syntax of the query varies a lot:

1. Select can query an expression, which is an operator + operand.

For example 1 + 1 2 * 3 2-1+5*9 math.random () * 1;

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As you can see, in the database, the string + string is not spliced, but the addition is calculated,

Try it and take ' 1 ' + ' A123 '.

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Expand, what if you want to make two strings of stitching?

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2. Aliases for tables

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In fact, the column can also alias:

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3. Use of the dual table.

Do we need to write out the name of the table when we query the expression?

No, it looks like this:

Select;

MySQL has a virtual table dual that complements the syntax of the query expression (the most appropriate syntax for those who have OCD!). ）

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Actually, do not add dual also line! Just for the integrity of the grammar.

When you query an expression, you can query more than one expression at a time.

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query expressions, which can be combined with the data in a table.

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Similarly:

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Advanced queries in MySQL