Which queries will return the total number of rows in a table?

 To retrieve the total number of rows in a table, different SQL databases have different ways to query this information. Below are common methods across various databases:

1. Standard SQL (Using COUNT(*))

SELECT COUNT(*) FROM table_name;

• This query returns the total number of rows in the table table_name. COUNT(*) counts all rows, regardless of whether they contain NULL values.

2. SQL Server (Using COUNT(*))

SELECT COUNT(*) FROM table_name;

• Same as the standard SQL query.

3. MySQL (Using COUNT(*))

SELECT COUNT(*) FROM table_name;

• Same as the standard SQL query.

4. PostgreSQL (Using COUNT(*))

SELECT COUNT(*) FROM table_name;

• Same as the standard SQL query.

5. Oracle (Using COUNT(*))

SELECT COUNT(*) FROM table_name;

• Same as the standard SQL query.

6. Using SHOW TABLE STATUS (for MySQL)

In MySQL, if you just need an approximate count (especially useful in large tables), you can use:

SHOW TABLE STATUS LIKE 'table_name';

This will return several columns about the table, including Data_length, Index_length, and Rows, which gives an approximation of the row count.

7. Using pg_class and pg_stat_user_tables (for PostgreSQL)

For PostgreSQL, you can query the system catalog to get an estimate of the row count:

SELECT reltuples::bigint FROM pg_class WHERE relname = 'table_name';

This provides an approximation, and the accuracy can depend on whether the table statistics have been updated.

8. Using sys.tables (for SQL Server)

In SQL Server, you can use system views to get an approximate row count:

SELECT SUM(p.rows) 
FROM sys.tables AS t
INNER JOIN sys.partitions AS p
ON t.object_id = p.object_id
WHERE t.name = 'table_name' AND p.index_id IN (0, 1);

This query provides an approximate row count by checking system tables.

9. Using INFORMATION_SCHEMA.TABLES (for most databases)

You can also query the INFORMATION_SCHEMA in most databases to get metadata about tables, including row counts (if available):

SELECT table_name, table_rows
FROM information_schema.tables
WHERE table_name = 'table_name';

Note: This method may provide an approximate row count and may not always be up-to-date, especially for large tables.

Summary:

• The most common and accurate method across all databases is COUNT(*).
• For large tables or when you need approximate values, you may use system views or metadata tables (SHOW TABLE STATUS in MySQL, pg_class in PostgreSQL, etc.).