What does * mean in mysql

In MySQL, the meaning of *

The asterisk (*) in MySQL represents "all". It has different uses in different contexts.

1. Select all columns

Use * to select all columns in the table:

<code class="sql">SELECT * FROM table_name;</code>

It is equivalent to writing out all columns in the table Name of:

<code class="sql">SELECT column1, column2, ..., columnN FROM table_name;</code>

2. Select all rows

In a subquery, * can be used to select all rows from the main query:

<code class="sql">SELECT * FROM (SELECT * FROM table_name WHERE condition) AS subquery;</code>

3. JOIN table

In the JOIN statement, * can be used to specify the connection of all rows:

<code class="sql">SELECT * FROM table1 JOIN table2 ON table1.id = table2.id;</code>

4. Wildcard character

In the LIKE clause, * can be used as a wildcard character, matching 0 or more characters:

<code class="sql">SELECT * FROM table_name WHERE name LIKE '%john%';</code>

It will match any row containing "john" in the name.

5. Regular expression

In the REGEXP clause, * can be used as a quantifier to match the previous pattern 0 or more times:

<code class="sql">SELECT * FROM table_name WHERE name REGEXP '.*john.*';</code>

It will match any name that starts or ends with "john".

6. Implicit conversion

In some cases, MySQL will automatically convert * to other types. For example, in a numeric context, it will be converted to a number:

<code class="sql">SELECT * FROM table_name WHERE id = 10;</code>

This is equivalent to:

<code class="sql">SELECT * FROM table_name WHERE id = 10.0;</code>

In summary, the asterisk (*) in MySQL means "all", in a different context have different meanings. It is typically used to select all columns, rows, or as a wildcard or quantifier in JOIN, LIKE, and REGEXP clauses.

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