What does _ in sql mean?

In SQL, the uses of underscore (_) include: 1. As a placeholder in query statements; 2. Used to escape special characters; 3. Identify SQL reserved words; 4. Create permissions Alias; 5. Concatenate columns into strings.

Underscore (_) in SQL

In SQL, the underscore (_) has several uses:

1. Placeholder

In the query statement, the underscore can be used as a placeholder to represent an unknown or variable value. For example:

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

2. Escape characters

Underscore can be used to escape special characters so that they have literal meaning in SQL statements. For example, to use single quotes, you can escape them as '_':

<code class="sql">SELECT * FROM table_name WHERE column_name = 'John''s';</code>

3. Identify reserved words

In some cases In some cases, you need to use SQL reserved words as column or table names. At this time, you can identify the name by adding a space after the underscore:

<code class="sql">CREATE TABLE _name (id INT, name VARCHAR(255));</code>

4. Restriction alias

Add a number after the underscore to create a restriction Right alias. Privileged aliases are only visible in the current query and will not affect other queries. For example:

<code class="sql">SELECT * FROM table_name AS _1;</code>

5. Concatenate columns into strings

In the GROUP_CONCAT() function, you can use underscores to concatenate multiple column values Concatenated into a string. For example:

<code class="sql">SELECT GROUP_CONCAT(_1, '_', _2) FROM table_name;</code>

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