PostgreSQL - Comparing Tables

Comparing tables in PostgreSQL can be done in several ways, depending on the specific requirements and the kind of comparison needed. Here are a few common scenarios and how you can handle them:

Check if Tables Have Identical Data:
If you want to determine if two tables, table1 and table2, have the exact same data, you can use the EXCEPT clause:
```
(SELECT * FROM table1 EXCEPT SELECT * FROM table2)
UNION ALL
(SELECT * FROM table2 EXCEPT SELECT * FROM table1);
```
If this query returns no rows, it indicates that the tables have identical data.

Find Missing Rows in One Table:

To identify rows that exist in table1 but not in table2, you can use a LEFT JOIN or NOT EXISTS:

SELECT a.*
FROM table1 a
LEFT JOIN table2 b ON a.id = b.id
WHERE b.id IS NULL;

SELECT a.*
FROM table1 a
WHERE NOT EXISTS (SELECT 1 FROM table2 b WHERE b.id = a.id);

Compare Specific Columns:

If you only want to compare specific columns, adjust your queries accordingly:

(SELECT col1, col2 FROM table1 EXCEPT SELECT col1, col2 FROM table2)
UNION ALL
(SELECT col1, col2 FROM table2 EXCEPT SELECT col1, col2 FROM table1);

Compare Tables' Structure:

If you want to compare the structure of two tables (i.e., columns, data types), you can query the information_schema.columns:

SELECT column_name, data_type, column_default, is_nullable
FROM information_schema.columns
WHERE table_name = 'table1'
EXCEPT
SELECT column_name, data_type, column_default, is_nullable
FROM information_schema.columns
WHERE table_name = 'table2';

Count Rows in Tables:
To simply check if two tables have the same number of rows:
```
SELECT 
  (SELECT count(*) FROM table1) AS table1_count,
  (SELECT count(*) FROM table2) AS table2_count;
```
If table1_count is equal to table2_count, the tables have the same number of rows.
Use Third-party Tools:
There are also third-party tools and software that can help you compare both the structure and data of PostgreSQL tables. Some popular tools include "pgAdmin" and "DataGrip". They often offer a more visual interface to help spot differences.

When comparing tables, always consider indexing and performance, especially when dealing with large datasets. Running comparisons on large tables can be resource-intensive and time-consuming.

Finding differences between two tables in PostgreSQL:
- Description: The goal is to identify rows that exist in one table but not in the other or rows with different values.
- Code:
```
SELECT * FROM table1
EXCEPT
SELECT * FROM table2;
```
Using EXCEPT to compare tables in PostgreSQL:
- Description: EXCEPT is used to retrieve distinct rows from the result of the left query that are not present in the result of the right query.
- Code:
```
SELECT * FROM table1
EXCEPT
SELECT * FROM table2;
```
Comparing tables with JOIN in PostgreSQL:
- Description: Using JOIN can help identify matching rows or differences based on a common key between two tables.
- Code:
```
SELECT *
FROM table1
FULL OUTER JOIN table2 ON table1.id = table2.id
WHERE table1.id IS NULL OR table2.id IS NULL OR table1.value <> table2.value;
```
Identifying matching rows in two tables in PostgreSQL:
- Description: Use INNER JOIN to identify rows that exist in both tables based on a common key.
- Code:
```
SELECT *
FROM table1
INNER JOIN table2 ON table1.id = table2.id;
```
Handling NULL values when comparing tables in PostgreSQL:
- Description: When comparing tables, be mindful of NULL values. You may need to handle them explicitly in your comparison conditions.
- Code:
```
SELECT *
FROM table1
WHERE (table1.value IS NULL AND table2.value IS NOT NULL) OR
      (table1.value IS NOT NULL AND table2.value IS NULL) OR
      (table1.value <> table2.value);
```
Performing table comparisons with WHERE clause in PostgreSQL:
- Description: Use a WHERE clause to filter and compare specific subsets of data between tables.
- Code:
```
SELECT *
FROM table1
WHERE NOT EXISTS (
    SELECT 1 FROM table2 WHERE table2.id = table1.id AND table2.value = table1.value
);
```