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:
SELECT * FROM table_name;
It is equivalent to writing out all columns in the table Name of:
SELECT column1, column2, ..., columnN FROM table_name;
2. Select all rows
In a subquery, * can be used to select all rows from the main query:
SELECT * FROM (SELECT * FROM table_name WHERE condition) AS subquery;
3. JOIN table
In the JOIN statement, * can be used to specify the connection of all rows:
SELECT * FROM table1 JOIN table2 ON table1.id = table2.id;
4. Wildcard character
In the LIKE clause, * can be used as a wildcard character, matching 0 or more characters:
SELECT * FROM table_name WHERE name LIKE '%john%';
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:
SELECT * FROM table_name WHERE name REGEXP '.*john.*';
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:
SELECT * FROM table_name WHERE id = 10;
This is equivalent to:
SELECT * FROM table_name WHERE id = 10.0;
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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